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 becomes 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. Collecting the secret eye causes it to show as it does in the editor if it has not yet been revealed.