The MOP is the Meta Object Protocol, which is the meta-language for controlling how CLOS operates internally. How exactly are classes, inheritance, instances, slots, method dispatch, method combination, etc implemented? In most object-oriented languages, you have very little insight into these issues and certainly no control over them. In CLOS you have both, thanks to the MOP. Just one more reason why CLOS is the most powerful OO system ever devised.