The CPU is the brain of your computer. It can do everything.

The GPU is a specialized idiot savant. It can only do one type of thing but it can do it really good.

The GPU is good at a certain type of math problem that is needed to create 3D images.

The CPU can do that sort of math too, but since it isn't specialized for it, it isn't as good at it. The CPU isn't as fast at that sort of thing.

The type of math the GPU does well is sometimes useful for other things too, like mining Crypto or certain types of simulations.