What is the "MMI"? 

MMI stands for 'Multi Media Interface', number and G means X generation. 
MMI consists of a single integrated interface, which controls a variety of devices and functions of the car.
MMI has COMPATIBILITY (not as default) Front / Rear Camera / Navigation compatibility by standard,

yet those accessories are considered as 'Optional accessories' hence not every model owns those.

Some Audi vehicles (Mostly base model) don't have MMI at all - It can be checked by the position of audio controllers. 
Car with MMI : Audio controllers are located around Gear Box.
Car without MMI : Audio controllers are located on audio unit itself.