The gap concept is quite simple and easy to understand.

In the gap concept it is stated that one player needs a better hand to play against another player who has opened the betting than the hand he would need to open the betting himself.

In other words the players avoid confrontations with players that have indicated their strength by opening the bets.

Just the same, calling only allows you to win if you have a good hand while opening /raising can get you immediate victory if your opponents choose to fold.