A subpoena is a notice that requires the receiver to make available certain evidence to be presented in court. (The word is also used as a verb to describe the process of serving the notice to the receiver).
This evidence may be that persons own testimony or it may be other evidence that they possess. (It is possible to subpoena documents for example.)

A subpoena differs from a summons in that usually a summons requires you to attend court in order to answer charges or complaints against you while a subpoena requires you to present evidence in a case against somebody else.