Let me try this one - the gap concept describes that to CALL A RAISE requires a hand that's STRONGER than a hand one would OPEN RAISE with in the same position. The difference between the strength of the two hands is the gap.

Say you have AJo in mid position. Everyone folds to you, so you raise. But if UTG raises before you, your AJ is probably no good any more as UTG must have a very strong hand to make this raise. So you fold.