The N64 Expansion Pak doubled the console's RDRAM to 8 MB. Certain later games, such as Donkey Kong 64, required the add-on in order to be played (Perfect Dark had limited functionality without it). Others simply saw graphical enhancements (e.g. finer textures, higher resolution).
The Expansion Pak was inserted into a slot on top of the console, replacing the Jumper Pak that came bundled with the Nintendo 64.