Definition of leak
A leak is a flaw in your game (or a flaw in anyone's game). A player can have several leaks. A leak is a feature that a player has that can be fixed. Most of the time, a player doesn't know about his own leaks (otherwise he would have fixed them). His opponents observe him, find his leaks and exploit them. There are an infinite number of leaks a player can have. Let's take an example. Let's say you are 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 plays fast with a nervous style, he's strong. That's obviously a big leak.
How to correct your leaks
In our opinion, there are 3 excellent ways to correct your leaks. The first (and easiest) is to hire a good poker coach. The second is to talk to your poker friends. Ask them to watch you play and critique you. Maybe they can find some leaks you weren't aware of. The third (and there are probably several others) is to analyze your statistics in your tracker if you play online. Look at your biggest losing pots, see if you find any patterns in your losing sessions, see what kind of hands are making you lose money, etc.