Definition of leak
An leak is a flaw in your game (or a flaw in anyone's game). A player can have more than one leaks. An leak is a trait a player has that can be corrected. Most of the time, a player doesn't know his own leaks (otherwise he would have corrected them). His opponents observe him, find his leaks's and exploit them. There is an infinite number of tens a player can have. Let's take an example. Let's say you're in a casino against a conservative player. When he plays slowly, talks slowly, makes decisions slowly, he has a weak hand most of the time. When he gambles quickly with a nervous style, he's strong. He's obviously a big leak.
How to correct his leaks
We think there are 3 excellent ways to correct these leaks. The first (simplest) is to hire a good coach of poker. The second is to talk to your friends about poker. Ask them to watch you play and criticize you. Maybe they can find some leaks that you weren't aware of. The third (there are probably many more) is to analyze your tracker's statistics if you play online. Look at your biggest lost pots, see if you can't find patterns in your losing sessions, see what kind of hands make you lose money, etc.