The gap concept
is a theory that originally was met by David Sklansky a top selling poker author and theorist.
The theory divulges into the requirements for calling an opening bet with a hand that would
not be able to open in the first place. Simplified, a player who opens the betting round
should only be called by players who have a better starting hand. The gap can either widen
or become narrow depending upon the play of the table. A loose player will make the gap
tighter while a tight conservative player will allow you room for a gap thus widening it.
How the Gap Theory Works
How this theory works is simple. A players who is sitting in late position holding J8 and has to call an opening bet from a person in early position should fold rather than call because the hand they hold can not compare to the hand of the opening bet. We know this because of the strength in the hand that the player in EP needs to have to do this. Thus making up the "gap" within the starting hand requirements.
Now if you were sitting in late position with either the hand folded to you could very easily throw out a bet based on that the other players hands are showing little to no strength to do this. However, if someone did call ahead of you your J8 is in most cases not even strong enough to limp. This type of positional bet is emphasized even by some of the worlds greatest who believe that if you are last to act and it is folded to you while holding small pockets....you raise. So far your hand is holding out strength, and if you are called by the blinds, place a continuation bet if the flop is trash. in most cases you will win an uncontested pot.
The gap concept relates only to players who understand this and know when their hands are folding hands. Mind you low limit tournament players don't pay much attention to this, but you will find it applicable in larger staked buy ins or tournaments that are nearing their finish. People start to respect a bet from the opponent who opens. It also can relate to why you will see a lot of blind stealing nearing the late stages of a MTT. If you are able to steal a blind in a tournament this equips you to see another round of free hands, and if it is played in position can justify others folding including the blinds who have no choice in the cards they have to pay for, making their hands weak in most cases.
If you want to advance to heads up position, using the gap concept will help you achieve this goal. It will reduce the amount of chips wasted making errors calling a positional player who has opened with a marginal starting hand. Always look at the table image of the player as well as what they could be holding to make that bet, then compare that to your hand value before deciding to make that call. A good poker player knows how to bet, but it takes a better player to know when to call and when to fold.