Signposts are objects that appear in various Mario games. They first appeared in Super Mario 64. They give you various information, such as how to do moves like wall jumping. Signposts also have names, like Phil Board and Bill Board in Super Mario Galaxy. Signposts also make appearances in the Paper Mario games and help out Mario by way of giving information on the basics of the game, directing to various locations, or hints on where to go next.
Super Mario Galaxy 2 introduced the TV signposts, which give a video tutorial on how to navigate a certain section of a galaxy or level.