A parody mass is a musical setting of the mass that uses melodies from other musical works. This kind of mass was popular in the 16th century. "Parody" in this sense does not mean "parody" in the modern meaning of the word. It is not about poking fun at something. A parody mass is one that borrows musical material. The music might be borrowed from a chanson or motet, usually one by another composer. At that time there was nothing shameful about using other composer's tunes. Today it would be called plagiarism (stealing other people's ideas).