From Quake Wiki
This is a solid brush with a few extra controls and characteristics. Basically, it has a targetname, allowing the mapper to manipulate it to some degree during play.
Additionally, because func_walls are brush entities, they do not create shadows and do not impact on vis compilation time.
Targetname - This gives it a name, so that other entities can trigger it.
If you put its targetname in the 'killtarget field of another entity (like a trigger_relay) then the func_wall will be removed when that entity is triggered. Typically a trigger_relay is used to do this, but be aware that it will not work properly under standard progs if the trigger_relay has both a target and a killtarget.
When killtargeted, the func_wall will vanish forever. Or until the map is restarted, or loaded to a time before the wall was killtargeted.
Swap its texture
If the func_wall has a texture named +0something then, when triggered by another entity it will change its texture to +asomething. See the texture naming conventions for more detail.
Bug: Flickering entity
This occurs when a large and/or complex brush entity crosses two or more vis leafs. It's a visual error whereby the entity will go invisible when the player is stood in a certain spot (ie. vis leaf).
The proper fix must come engine-side, the only mapside fix that can be done is to divide the func_wall into smaller ones that do not cross leafs.