Two important things to know before getting into pass protection and the run game are the systems teams use for identifying each gap and for identifying the alignment of each defender on the line of scrimmage (LOS).
The first system is for identifying each gap, which is useful when talking about pass protection and also the run game.
As the above image shows, the A-gaps are the ones in-between the center and each guard. The B-gaps are the ones in-between each tackle and guard. The C gaps are the ones in between the tight end and tackle, and if there is no tight end it’s just everything outside of the tackle. The D-gaps are the ones outside of the tight end.
The second one is for identifying the alignment of each defender on the LOS. Each alignment is called a technique, so for example, “Nick Bosa is in a 9 technique”.
The only confusing one is the 9 technique, which is on the outside shoulder of the tight end, as there’s no logical reason why it shouldn’t be 7, but that’s just the way it is.