The Sign is a craftable Structure. While it can't be written on, it shows up as an icon on the map. As such, it can be used as a marker for areas of interest which don't show up on the map (such as Blue Mushrooms or Beefalo herds).
The Sign requires 1 set of Boards to craft and a Science Machine to prototype. If it is destroyed, the Boards are returned, allowing the player to build a new Sign elsewhere.