An important component of good defense in football is understanding your gap assignments. Your coach might tell you, if you're playing linebacker for example, that you drop in zone coverage on pass, and you have "B" gap if it's a run play. So how do you know which gap is which? Here's a simple description and illustration to help you understand gap assignments.
How Are the Gaps Labeled?
When the offense is in a standard formation, you start at the center and work your way out in both directions. The space between the center and guard is the "a gap" on both sides. The space between the guard and tackle is the "b gap" on both sides. The space between the tackle and tight end is the "c gap" on both the right and left side.
Keep in mind that the gaps do move as the play develops. You may have a larger territory to cover if you've got "c gap" and the tight end moves to the outside. It's still your assignment, and your responsibility to cover that gap. Understanding and recognizing these gaps will help you stick to your assignment and make big plays.