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 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.
The U shaped/tuning fork pillars: feel your controller - it's vibrating in a pattern.