Metamod:Source is a C++ plugin environment for Half-Life 2. It acts as a "metamod" which sits in between the Game and the Engine, and allows plugins to intercept calls that flow between. It provides a mechanism called SourceHook, a very powerful library for intercepting, overridding, and superseding virtual function calls.
METAMOD for CS2 can be conveniently installed in the webinterface'
Metamod is required for every Plugin you want to use on your CS2 Gameserver.
more about MM you can find on the official page