MOP. (Whatever that is.)

It stands for "Meta Object Protocol", it is basically the API to the object system itself. Why not just say "API to the object system", well, the term is taken from CLOS (Common Lisp Object System) and Class::MOP has more of a ring to it then Class::API::To::The::Object::System::I::Just::Wrote. If you want to learn more about MOPs, you can read The Art of the Meta Object Protocol online, it is an amazingly eye opening book, the first three chapters are well worth reading through.