Refers to the original BMW M3 (e30 chassis) which won countless touring car championships in the late 80's and early 90's. This car was offered in a street version as well, touted as one of the best handling vehicles ever available to the public. A race car for the street if you will.
Not to be confused with later BMWs sharing a similar name (in the E36 and E46 versions the M stands for Marketing, not Motorsport) the original M3 was, is, and always will be the most badass car to ever have existed.
Did you see that old-school M3 smoke that 996!?!?!?! WTF how can an old car pull through a turn like that!!!!