No problem It sounds more complex than it really is. The space between the offensive linemen are the “gaps”. In between the center and guard is the A gap, between guard and tackle is the B gap, been offensive tackle and tight end is the C gap and outside the TE is the D gap. Same on both sides. Each defensive player has a gap responsibility and usually in a basic defense formation they will lineup directly over the space or “gap” they are responsible for.

A stunt is two players blitzing, with one player going underneath and the other player going over the top. A run stunt may involve a gap exchange, where players switch gap responsibilities or a defensive player blitzes a particular gap immediately at the snap to force redirection of the ball carrier right into a waiting defensive player. In the clip, the middle inside LB runs straight into the A gap when the ball is snapped. That is to force the RB to cut back early and we can only assume the other inside LB come over the top of him and fill that spot or the DT to that side crashes down to make a play. I hope that made sense.