A "false wall" is a two-sided linedef with a visible middle texture, often with the lower unpegged flag on and the impassable flag off. An example can be found in Doom's E1M1: Hangar where it is impossible to see inside the secret passage from the zig-zag room, while it is possible to look out of the passage while being in it. It is recommended to use a different flat or other property for the sector behind the false wall linedef. If said sector has no property changes (ie., a different flat, light level, or height value) compared to the sector in front of the false wall, the middle texture will bleed into the floor, ceiling, or both, especially if the sector is less than 128 units high and the texture is 128 units in height. The false wall linedef can also have a texture on both sides.
Creating a false wall
Create a sector and make another sector connect with it. Highlight the linedef in between the sectors. Open up the sidedef textures menu and put the texture you want to use in the middle texture slot. In case you want to make a false wall with a texture only on a single side as in E1M1, you need to pay attention to the orientation of the linedef to get the middle texture on the side you like; the front or first side of the linedef is the one with a small line sticking out from its center while being in the editor view. If you instead intend to use the linedef as a solid gate or as a masked window with middle textures on both sides, remember to make it impassable via the linedef menu.