This is a colloquial term for using defensive attack sequences to ward off and hit enemies attempting to advance into range. For example, the neutral air attacks of Mario, Dr. Mario, and Luigi are excellent walling attacks if you jump, stick them out, then fast-fall down. The lengthy active period of the kicks covers the whole fast fall duration. Another example of this kind of tactic involves jumping and double-jumping while sticking out a quick high-priority air move over and over, like Mario jumping with his back to foes and repeatedly poking with back air.
While it isn’t easy, these kinds of tactics can be punished by fighters with fast movement, usually by rushing in rapidly, shielding the air attack, then shield-grabbing as the stalling fighter lands.
Occasionally, walling is also used to describe repeatedly throwing out a fast ground-based attack to act as a checkpoint against overly aggressive opponents.