They are referring to the alignment, in respects to where the O-Lineman are. When you are talking about football, defensive and offensive terminology are not the same. The offense numbers the gaps and the defense letters the gaps. For example, to the right side of the center the gaps are even numbered 2,4,6,and so on. To the left of the center the gaps are numbered odd 1,3,5, and so on.
The defense on the other hand letters the gaps. For example, on either sides of the center are the A-gaps. The outside gaps of the guards are the B-gaps. And the out-side gaps of the tackles are the C-gaps.