This refers to any barrier that requires the heroes to solve some kind of puzzle or Riddle Me This in order to pass. Alternatively, it could be some test of skill. Often given by Threshold Guardians (especially if the guardian in question is a Riddling Sphinx). You must be able to work it out based on the clues you are given on the spot. (If you must come to it with some knowledge, it's Only the Knowledgable May Pass.) One wonders why this was deemed a better barrier than say, a lock and key carried on someone's person instead of being in a chest elsewhere in the building.