I'm no guru and I have no college degree, so anybody else correct me if I'm wrong, but iirc thats where the processor stores whatever information it is currently processing. I guess you can call it the "fastest" memory in your computer. When you see a "32-bit" or "64-bit", etc, processor, that refers to how large this specific memory segment is. The larger it is, the more information the processor can crunch per clock cycle.