Wallshooting is one of the most important mechanics to get your head around for speedrunning in zombies. Wallshooting means shooting your gun through a wall to pierce a mob on the other side of the wall. This typically requires shooting through a non solid block such as a stair or slab, which will not stop your shot until it hits a mob in that route.

Here is an example of a wallshoot from mansion to kill mobs that often get stuck by the crypts stairs:

You can see the gun is being aimed through the stone brick stair, so the bullet will not be stopped

Wallshoots can be aimed where mobs spawn in windows, where specific mob spawns points are (e.g. herobrine minions, blazes, skeletons etc.), at moving mobs (by aiming in a path you predict they will go) etc. When you aim at a spawn point you typically shoot when the timer hits the spawn time (or just before, called pre)

It is quite inconsistent what blocks you are able to wallshoot through but here is a brief list; all stairs (other than dark oak and spruce), all slabs, barrier blocks, iron bars, fences, chests, and signs.

When wallshooting you don't have to be looking exactly on the block if it is not a full block, for example this wallshoot from office corner to an alley spawn window will hit:

As you can see I am not really aiming through the slab itself, but I am aiming through where a full block would be so it still will not block the shot, here is another image to understand this: