Eagle-eyed members of the Thieves Guild may notice certain locations across Skyrim are marked with strange little pictorial signs. These inscriptions, which are unknown markings to outsiders, are actually a cryptolect that the Guild uses to flag locations that members feel are particularly safe or dangerous or that have entities that may be helpful or problematic. The symbols are known as Shadowmarks.
To learn more about Shadowmarks, search the Thieves Guild premises for a particularly well-researched book called Shadowmarks, written by Delvin Mallory. It contains all the markings the Thieves Guild uses.
