The tournament poker concept of the gap was coined first by David Skalansky but most good poker players have a feel for this intuitively even if they have not read it. The idea of the gap is that you need better cards to re-raise than to open a raise. That is, you may be willing to open the raising with AJs but if there is already a raise in front of you AJs becomes a lot worse. In fact, depending on the size the bet you may not even call. The idea is that the worth of your hole cards depends a lot on the action before you and that if there hasn’t been any sign of strength yet, then the range of cards with which you can raise is wider. Further, there is a “gap” between the hands with you can raise with and the hands you call a raise with.
The concept of the gap is most important for tournament players. This is because ring games necessarily allow players to be tight but in tournaments the blinds will eventually force players to become more and more aggressive. So you need to have a feel for when you can raise and what you can call a raise with. The other reason the gap is important for tournament players is that, in tournaments blind stealing is so important. Especially in the late stages of the tournaments where blinds are large and players are forced to ante as well, the pots even before any raising can be a significant steal. Basically, every time you steal the blinds, you allow yourself at least ten more orbits. So while the aim of stealing the blinds is keep you around long enough to get a decent hand, the actual act of stealing the blinds doesn’t need that strong a hand , a la the gap theory. Likewise, knowing this, when you are in the blinds, you should consider the gap when working out whether you defend your blinds from a suspected blind steal.
At the fundamental levels, the reason the gap makes sense is that a raise from early position carries more weight than a raise from late position. Raising under the gun is totally different from raising from the button. Players are generally tighter the earlier their position is because they are necessarily making decisions with less information and more risk. That’s why a raise from early position usually means more.
The reason this is important in your own hand consideration is that you want to avoid going into hands where you are in a dominated situation. That’s why AJs might be fine to raise with but suddenly calling a raise from UTG is totally different. Now you are at real risk of being totally dominated by a hand like AK or AQ or worse. This example is a good illustration of how understanding the gap can help you avoid situations where you are dominated, this should eventually realizes itself in your EV as you will be avoiding some big losses.
The final important point is that, you need to adjust the gap according to the player. The crazy wild man who raises with whatever two cards in effect nullifies the concept of the gap. But this is an extreme example but illustrates that concept of the gap depends very much on the players. A lot of people are arguing that Sklansky’s concept of “the gap” is as antiquated as Sklansky’s starting hand selections. That the hyper-aggressive nature of modern tournament players, where it is always about playing the player rather than the cards. With more fearless re-raising, that over-valuing ideas concepts like the gap are likely to lead poker players to play too tight and be blinded out. This may or may not be true, in the end the gap was always supposed to be adjusted accordingly to the player.
Of course, tournament players should not over-think the gap to the point where they are playing too tight. In the end it just a general framework to mentally grasp the relationship between your hole cards and the others in the end. That is, the concept of the gap has value, it is just a matter of being efficient and not over or under valuing it.