A hidden wall is a tile that acts exactly like a wall, but initially has the appearance of a normal floor. When Chip touches it, it is revealed and looks like a normal wall. Monsters and blocks will not reveal a hidden wall, although they are still subject to it.
The image above is the symbol shown for hidden walls in level editors; note that in the actual game, the hidden wall looks identical to floor when not revealed and identical to a wall when revealed.
Note that if a hidden wall is revealed on the lower layer of the grid by Chip under a removable tile or floor, it will also appear as a normal floor space until touched.