That is your northbridge heatsink, and it is in fact there, to keep your northbridge cool. Specific reason you're asking?
Teacher keeps calling it a cpu -_-...
Haha that's great, I'm in a college level computer class and my prof continues to tell the class that the ram is faster than a hard drive because it's closer to the cpu. It makes me want to cry/laugh. Please correct your teacher. The big brown chip with AMD written on it is your CPU.