Ancient walls are advanced devices capable of concealing areas of a structure from sight and sensors. The wall itself can be made non-solid or, in later versions, even disappear when instructed.
A similar wall hides the enterance to a hidden Wraith laboratory used for prohibited genetic research. It is unclear if this uses the same technology, or was invented independently.
Two walls have been seen so far, these are:
Janus used a wall to hide his secret lab whilst conducting research on various technologies banned by the Ancient Council. The wall was located in Atlantis and was triggered by a series of sounds produced by tapping the 3 lights on the wall near it. It became non-solid allowing Janus to access his research without fear of being discovered. (SGA: "First Contact")
Morgan Le Fay's Wall
Morgan used a wall that was impervious to sensors aside from a minute energy signature that is barely detectable even to those looking for it. It was used to protect a tunnel in a cave. The wall was shaped as a rock face that vanished completely when the hidden button near it was pushed. (SG1: "Morpheus")