When someone improves or modifies the game we call his work a Mod.
So MedMod is another way to play CTP2. When you set it up you can choose if you want to play the medmod or the original game. Playing the medmod you get new features to the game like 2 new governments, 50 new units, 60 new advances, all the bugs fixed and more.
The medmod isnt the only mod around here. We also have Craddle and Apolyton with their differents features and modifications. To get informations about those go to the CTP2 Creation/AI/Mods/Scenarios forum read their respective threads and the others threads also.
I hope i cleared things up