In fez, there are several pillar/monoliths scattered about the world. When Gomez approaches them, dot will say things like "I wonder what this is" or "I forget what these are". So far they have been in all of the areas that still have questions marks for me. What do they do?
There are a couple of different types, each with a different solution. You'll figure them all out eventually, and doing so is part of the joy of the game, so I'll provide hints and the complete fix as spoilers.
The thick pillars with vertical glyphs on them: in the library/observatory area there's a room with one of these pillars and a part of the level that responds to your controller inputs.
The vertical glyphs are a line of tetris symbols on its side. Each symbol corresponds to a controller input: the 4 'T' blocks are cursor directions, the square is a jump, and the 'S' and 'Z' blocks are RT and LT respectively. Usually completing these gives an anti-cube, but sometimes they open a door.
The tall thin pillars with a square at the top: you need to be in new game+ before you can solve these. Once you're in NG+ try a different perspective.
Stand on the pillar and switch to first person mode, you should see a tetris symbols glyph nearby. Completing these gives an anti-cube.
The U shaped/tuning fork pillars: feel your controller - it's vibrating in a pattern.
The left and right sides of the controller vibrate the pattern you should repeat with LT and RT. Completing these gives an anti-cube.