The ruling was that the FBI request directed at Apple to unlock an iPhone was dismissed as moot when a third-party contractor for the FBI figured out how to access the locked iPhone, so the legal issue wasn't actually resolved on the merits in that case. And, since the method used by the contractor is widely applicable to many situations where this could come up, it is unlikely to be resolved soon on the merits. A timeline of the litigation with references that would allow you to do further research if you wished is here.
Thus, as it happened, the answer is boring. It was a case that might have been important, but wasn't. Because the issue was not resolved on the merits, the decision has no importance as law of the case, case law, or under other preclusive doctrines, except on the narrow legal question of under what facts a legal dispute becomes moot.
Law of the Case
Law of the case is a legal doctrine with two components.
One says a trial court may follow its own prior rulings on the same legal issue in the same case if it is raised again (and will do so unless there is a good reason not to), without reconsidering it on the merits.
The second says that a trial court must obey any appellate rulings previously entered in the case (the second part of the law of the case doctrine is also known as the mandate rule). Law of the case does not apply to the question because it was not previously resolved in the same case.
For example, suppose that the court had ruled that the FBI could not order Apple to crack the first iPhone. If the police later found another iPhone and asked the court to order it opened, the court could apply the law of the case to say: "No, my ruling is the same as the last time you asked."
But, if the prosecution had convinced the court that it has made the wrong decision previously, or that circumstances had changed due to a new law, or a new precedent, or new technological information that the court didn't know about last time, the court would have the power to reconsider its own prior legal ruling on that issue.
Case Law Distinguished
Case law is guidance regarding how a court should decide a case in a common law legal system based upon decisions in previous, unrelated cases presenting a similar legal issue, according to the principles of the doctrine of stare decisis and the hierarchy of the court system.
Rulings on legal issues which needed to be made by a court which has appellate jurisdiction over the court considering the legal issue that have not been overruled are binding authority that the lower court must follow unless an exception apply.
If the ruling in the precedent is on an issue that didn't have to be resolve on the merits in the case before it, it is non-binding dicta and is only persuasive authority.
Exceptions to the rule that case law is binding exist (1) if the facts of the case before the court considering the legal issue are distinguishable in some way that makes the precedent irrelevant, or (2) if subsequent changes in socio-economic reality makes the legal analysis used unsound, or (3) if the statutory language relied upon has changed or been interpreted differently, or (4) if the way that similar but not identical issue are now analyzed in cases that are binding precedents suggest that the precedent is no longer an accurate statement of the case law even though it has not been expressly overruled by any court. Several additional rules exist to resolve apparent contradictions between seemingly binding precedents.
Dicta in cases that would otherwise be binding precedents, and cases decided by courts that do not have appellate jurisdiction over the court considering the legal issue, and sometimes, unpublished decisions of courts that do have appellate jurisdiction over the court considering the issue (the status of unpublished decisions varies from jurisdiction to jurisdiction), are persuasive authority that can be deviated from by the court considering the issue but usually aren't unless the court considering the issue things that the sister court's decision was ill reasoned or unwise or contrary to binding precedents.
Case law does not apply strongly in this situation because it presented an issue of first impression never authoritatively decided by any other case in a highly similar fact pattern. This is why the case was interesting.
For example, if this had not been an issue of first impression, the prosecutors and defense attorneys and the judge and the judge's law clerks would do legal research to try to locate any binding precedents telling the court what to do, and if none could be found, to find any other cases that were not binding precedents, or any secondary sources (e.g. law review articles) that could provide persuasive authority, and would consider that persuasive authority while coming up with a new rule for the case of first impression.
If the prosecution or defense thought that the decision was wrong, in an evidence suppression, pre-trial order to a third-party case like this one, there would probably then be an interlocutory appeal (i.e. an appeal before a guilt or innocence determination is made or a trial is held) to the relevant appellate court(s) to resolve the issue (and possible even from there to the U.S. Supreme Court). If those courts made a ruling, the trial court would then enforce that ruling on remand pursuant to the mandate rule.
Other Preclusion Doctrines Distinguished
For completeness, there are also several other doctrines similar to law of the case and case law that also do not apply. When these doctrines apply they are binding and not discretionary guidance for a court considering a legal issue or matter.
Res judicata also known as claim preclusion applies when a dispute between the parties or closely related parties has already been litigated and prohibits reopening the dispute.
For example, if, after the federal government won in this dispute, Apple filed a new lawsuit related to the same particular phone that the FBI was trying to crack the first time, then res judicata would bar that lawsuit by Apple from being tried since the case had already been litigated, even if Apple came up with a new legal issue that it hadn't considered in the first case.
Collateral estoppel also known as issue preclusion applies when a legal issue or mixed issue of fact and law was resolved in previous litigation involving the parties or closely related parties in a different matter, and prohibits relitigating the issue previously resolved.
For example, if the federal government has previously lost this issue in another case against Apple, that ruling would be controlling of the result in this case (subject to various exceptions), and the federal government would lose without reconsidering the merits of the legal issue presented vis-a-vis Apple, since Apple and the federal government had already litigated and resolved this legal issue.
Double jeopardy which applies only to criminal law case and is similar to the doctrines of res judicata and collateral estoppel in some respects, prohibits the retrial of cases where a trial was commenced and the defendant was acquitted or the case was dismissed due to a mistrial that was the fault of the prosecution. Obviously, there are nuances to it that are beyond the scope of this answer.
For example, if the defendant in this case was acquitted of whatever crime the iPhone was being probed in hopes of convicting him of, he couldn't be prosecuted for that crime again, or for any crime for which a conviction would be logically inconsistent with his acquittal. But, if he was prosecuted on some other unrelated charges to which the iPhone was relevant, or in a state court (the original case was in federal court), double jeopardy wouldn't apply (and neither would law of the case) even though another preclusive doctrine or case law from the original case might be pertinent.
None of these legal concepts, sometimes called preclusive doctrines, actually have application to this case because there are no prior cases that addressed the same legal issues or disputes arising between the same parties.