Supererogation is a fancy way of saying "doing more than you're expected or obligated to." If your school requires that all students perform ten hours of community service, but you volunteer at the soup kitchen for twenty hours, that's supererogation.
The Late Latin supererogatio means "a payment in addition," from super, "above or over" and erogare, "pay out." In English, this word was originally used in a religious context, for good works that go beyond what God requires. Today, supererogation typically describes an ethical or religious decision to act beyond what's required for being a good person: "Donating twenty percent of their income to charity is an act of supererogation."