When there's enough of something, you don't need or want anymore. When there's not enough, there's too little.
If someone fills your glass almost to the top, that's enough. If you ate one slice of pizza but are still hungry, you haven't had enough. Although enough has to do with the amount of something, it isn't precise like a number. When someone is annoyed — like with a barking dog — they might say "That's enough!" When you've had enough, you've had as much as you want.