A subpoena is a document that requires its recipient to appear in court as a witness. If you receive a subpoena, it doesn't mean you've done anything wrong; it just means you may have information that's needed by the court.
Subpoena can also be a verb: You can subpoena someone by giving them a subpoena. If you receive a subpoena but fail to carry out its instructions, you're in big trouble. This fact is suggested by the Latin roots of this word: the prefix sub- means "under" and poena means "penalty." One nice thing about a subpoena is that it's not a summons; if you get one of those, it means you're being sued.