Compression is just that. How soft are hard it is to compress. To hard it feels to hard and the shock doesn't work well. To soft and the bike just bottoms out and feels off.
Rebound is the speed the shock "resets" to ride height.
To slow it packs. Meaning it bottoms out and you basically have no shock.
To fast and it starts to buck. Meaning it shoots back into position in a violent snap. That can send the bike launching forward or just bounce the front end around.
But there's a lot more to it. Spring rates and ride height/preload there's high speed and low speed compression.
Any more then that, is above what I know.