I can drive, listen to music, and have a conversation but it starts to overload my brain at that point, as most of my attention is focusing on driving.

The usual computer analogy is multitasking, in which the kernel rapidly switches from one process to another. A single CPU core may switch between running code for my browser, my chat program, the temperature monitoring process, and the wifi driver; this happens so quickly that it appears to me that all of them are running “at the same time”.

Attention also shifts quickly. When you are doing two things “at the same time”, attention is switching back and forth between them! You’re not really constantly attending to the music and the road; ideally you switch back to the road often enough that if something surprising happens there, you can respond to it in time.

We know from experiment that when people have more distractions going on while driving, they actually do respond slower to surprises on the road. Eating a bagel with the radio on and your kids in the back seat is actually hard, and really does slow down noticing the dog that just ran out into the road.