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 of this can be found in Doom E1M1: Hangar: 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 for the sector behind the false wall linedef (if the same flat is used, 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. If you intend to use the linedef as a solid gate, make it impassable via the linedef menu.