The Gap Concept was first introduced by professional poker player and strategy book author David Sklansky. It is a very simple concept but one that many beginners, and indeed, some experienced players fail to grasp or choose to completely ignore. It is one of the first pieces of theory you should commit to memory if you want to learn texas holdem poker.

It revolves around a basic statement that a players needs to have a better hand to play against someone who has already opened the betting, with a raise, than they would need to open the betting themselves. For example, if you are seated in middle position and the action folds around to you, it will often be correct to raise a hand such as Ace-ten offsuit but if a tight players raises from early position it will nearly always be correct to fold that very same hand.

This is because when an early positioned player opens the betting with a raise, they are indicating they have a strong hand, one that is strong enough to beat the hands of everyone else seated at the table. A raise in early position is usually given more credit than a late position raise for this reason.

When someone opens in front of you, you should tighten up significantly  for two clear reasons. The first is that your opponent has already announced he has a hand he believes to be strong enough to defeat the entire field plus you actually lose some credibility in the hand when you simply call the raise as you will not be believed when you attempt to show strength on later streets.

Another very real risk is of your hand being dominated by the initial raiser. Imagine again we are in middle position with our Ace-Ten offsuit and we decide to call a tight player's raise from early position. Some of his possible holdings such as Ace-King, Ace-Queen and Ace-Jack all have our hand completely crushed and if we flop top pair we could lose a large pot with what we believe to be the best hand.

The size of the gap is not static, in fact it alters with every player. If your opponent is extremely tight than the gap becomes much wider but if the raiser is loose and aggressive you can shorten the gap and call them more lightly.