Yes, defenders have gaps they are responsible for. Against some running plays, this is pretty straight forward. The RB is supposed to run to a specific gap. Against zone running plays or plays with pulling linemen, it's somewhat less straight forward. The RB isn't really told what gap to run to. He's given a general area/zone and told to read what comes open. When running outside zone, the "gaps" tend to naturally get wider. The gap a defender was supposed to defend based on the play call could be 5 times the size it was when the play started. If an OL pulls into that gap he's essentially split that gap into two "gaps." In that instance, the defender has to try to slow play both gaps while someone can chase the runner down from the sides. You can't pick one and leave a new "gap" that's bigger than the one you were initially assigned to defend. That's why pulling lineman are so prevalent now. They essentially create new gaps to defend that no defender was initially assigned to.
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