also: seal · wall · cut off
Getting in a defender's way so he cannot get to the ball. There is no requirement to move him, and downfield it is closer to setting a pick than to blocking anybody.
2 meanings, all below. Jump to the one you heard.
To wall off is to get in a defender's way. It literally means that, and the plainness is the point: there is no requirement to move him and no requirement to win in any conventional sense. A defender who is standing still on the wrong side of a blocker has been handled completely.
The finish names itself. The blocker has turned the defender so his back is to the ball and he is looking away from where the play is going, and a defender in that position cannot pursue without first turning around. This is why the word appears constantly in backside rules, where the honest instruction really is to be in his way.
seal, cut offpin, log block, base blockDownfield is where walling off stops being a lesser version of blocking and becomes the only thing that works. Once the back has hit the hole with speed, the job is to set a pick and get the second-level defender separated from him. That is all of it.
Trying to deliver a huge block out there is a bad bet for two reasons at once. The defender is trying to avoid you rather than take you on, so there is often nothing to hit. And the back may not be where you think they are, so a blocker who commits to a big collision can win it and still have walled off the wrong piece of ground. Staying between the defender and the ball keeps working even when both of those go wrong.
stalk, screen him offcut block, climbThe highest-value thing to tell a young blocker, because the instinct is the opposite one. A player who believes blocking means delivering a hit will chase a defender who is avoiding him and take himself out of the play, when standing in the way would have finished the job.