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. This type of masked wall must be used to make mirrors.