There are a lot more differences than this, but if your teacher sums up what he/she means by that sentence.
Here, specifically, in Civil Law, the decision of the courts must comply with the laws as enacted, which means there are specific statutes required to make something illegal.
Common Law features Stare Decisis which basically means that if Case A is decided in one way, and Case B is a similar Case to Case B, Case B must yield the same decision for all cases in that jurisdiction and lower courts below that court. This means that while statutes (laws) can be made by a legislature, the courts can "make law" by deciding cases. For example, some States in the United States and England and Wales only recently (within the past 30 years) adopted an actual law that made murder illegal? Prior to that murder was illegal under Common Law Murder that had been based on precedence from bazillion cases before that said it's illegal. Nobody bothered to write it down in an actual law.
There are several other big differences such as Inquisitorial vs. Adversarial nature of courts, how and when punishments are decided (The famous "Just following Orders" Defense was given in part because of this difference and a lack of understanding over it.), who is the trier of fact vs. who is the trier of law, but as far as what is "Law" this is a good single summation of the difference in a single sentence. But it really shouldn't be condensed to a single sentence.