A subpoena is nothing more than court process compelling someone to testify as a witness or to produce documents in their possession, custody and control, or both, usually in connection with a court case.
Most people who are subject to a subpoena are not being asked to offer testimony that could implicate them in a crime. They simply have evidence relevant to a case.
For example, bank records are routinely subpoenaed to generate evidence that can be used in a civil or criminal case against someone, even though no one accuses the bank of doing anything improper.
Many witnesses are also just not bothered to testify or actively don't want to testify, even though the evidence would not personally reflect poorly on them in any way, if they are not compelled to do so.
For example, they may simply be very busy at profitable activities, or may fear retaliation from people involved in the case.
Also, subpoenas are not infrequently issued where the witness is personally happy to testify but needs court process to get permission to be away from work or other obligations.
A subpoena can also overcome legal obligations to not voluntarily provide information even when it is not legally privileged. For example, a lawyers ethical obligation not to voluntarily share information about a client (even information that isn't attorney-client privileged like the existence of an attorney-client relationship or the amount of money that the attorney holds in a trust account for the client's benefit) is broader than the attorney-client privilege and a lawyer can be compelled to provide such information by subpoena.
To invoke the 5th Amendment in a case where you are not a defendant, you must have some good faith belief that your testimony would expose you to criminal liability, something that the vast majority of witnesses under a subpoena do not.
You cannot invoke the 5th Amendment, for example, to protect yourself from having to testify regarding something that may be highly embarrassing and may even constitute a violation of the law, but is not a crime.
For example, you can't invoke the 5th Amendment to prevent yourself from having to admit under oath in court testimony that you cheated on your boyfriend with someone else, or that you forgot to lock up the office the night before it was burglarized, or that you lied about having won a military decoration that you publicly claimed to have won but really didn't, or that you are in default on your mortgage, or that you are out of legal immigration status.
The 5th Amendment can also not be invoked to prevent you have having to testify about violations of professional ethics that are not crimes. For example, an attorney could be compelled to testify that she failed to keep confidential information private by telling her bartender client secrets.
A subpoena is basically useless against a defendant in a criminal case, who has the right not to be compelled to testify in his or her own case.
But, in a civil case, you can subpoena someone to testify against themselves and they then have the choice of invoking the 5th Amendment and having an adverse inference entered against them (usually resulting in them losing the case if they are a party to it), or not invoking the 5th Amendment and testifying even though truthful testimony on the subject may be incriminating and may be used against you in a criminal prosecution.
Someone can also be compelled to testify regarding matters that could be self-incriminating in a civil or criminal case if someone with authority to do so (generally the relevant prosecuting attorney) grants the person compelled to testimony immunity from prosecution (usually all that must be granted is immunity from prosecution based upon the testimony offered, called "use immunity", which is still less than someone admitting to criminal conduct might need to feel comfortable if testifying voluntarily).