The Gap Concept was pioneered David Sklansky, one of the world's premier poker writers. Many of his ideas have had an a big impression on the poker community as a whole. The Gap Concept is quite a simple idea though; it means "you will need a better hand to call an opening bet than you would to open the betting yourself". Therefore if you were first to act with AQ suited, you could then open strongly with a good size bet, but if you had to respond to a bet of that exact size in later position holding AQ suited, then the correct course of action is to fold. You'll see that this concept is particularly important in tournament play
The difference between the strength of the hand needed to call the opening bet and that of the hand needed to make the opening bet in the first place is "the gap". This gap is not solid or fixed, it depends on the type of opposition you are facing. If they are loose then the gap concept narrows, if you face tight players then the gap concept widens i.e. you should only play with a very strong hand. The gap concept is also influenced heavily by the size of your stack, but as a general rule it may be wise to open the betting holding pocket deuces but it is nearly always foolish to call that bet with the same cards.
It should become clear that the gap concept can be used to your advantage in early position. Being under the gun (first to act) you'll be able to ward off any hands that are similar to yours by making an aggressive bet. Most people acting behind you should fold to this early aggression.
However be warned, for the gap concept to work players have to be aware of it! There fore it may be trickier to use it to your advantage when playing online. The average newbie online either doesn't play according to position or hand ranking or simply doesn't understand the importance of it. They would typically play A10 the same way whatever their position was pre-flop.
The Gap Concept works best when you can see that the other players at the table are familiar with it, and are visibly applying it to the game. The general advice is to give people the benefit of the doubt in live poker, play as though everyone is aware of the gap concept until you get evidence to the contrary. If people are consistently not applying the Gap Concept then start loosening up your play and your starting requirements from certain positions. Good luck at the tables!
For more of David Sklansky's concepts and ideas read The Theory of Poker