What is a gap in football?

Answer 1

Gaps correspond with the areas of space between offensive linemen. Each gap is given a letter: the space between center and guard is the A gap, between guard and tackle the B gap, between tackle and down tight end the C gap, and any space past an uncovered tackle or tight end the D gap.

Answer 2

The gap between the center and guard would be the "A" gap. These gaps are assigned when calling a play to notify the players where a running play is designed to go.