Gap conceptThe gap concept was formulated by David Sklansky. He says that it takes a stronger hand to call a raise than to make it in the first place. There is a gap between the hands with which one could raise and the ones with which one could call a raise.
This is a description of the fact that one could go all-in with many more hands than one could call an opposing all-in in the late phases of a tournament. Likewise, there is a larger range of hands with which one can steal the blinds or antes than with which one can defend them.
A consequence of the gap concept is that aggressors always have an advantage.
Blindsteal, First in vigorish