A One-Sided Masked Wall is only visible from one side. It is usually referred to as a "One-Sided Wall".
In 3D mode, point the mouse cursor at either the floor just below or the ceiling just above any two-sided wall, and press Shift + M
.
A one-sided masked wall will appear. These are useful for hiding secret
places.
You can also make a 'solid' one-sided masked wall with the 1
key instead. The only difference is that this type of wall will not be compatible with transparency settings or invisible (pink) pixels. These are usually used to make mirrors.