Rift Signature is an item added by Dimensional Doors. It is the weaker version of the Stabilized Rift Signature. Rift Signatures are used to open rifts in the overworld that can be traveled through with the use of a Warp Door. Simply right-click with the Rift Signature at one location, then at the second. This should open the rift. Place the Warp Door on the rift, and step through it. This should instantly transport you to the other location and make a second Warp Door at that location. It can only be used once however, unlike the upgraded item Stabilized Rift Signature.
These rifts may be used cross-dimensionally, from the Overworld to the Nether, The End, or other Dungeon or Pocket Dimensions, and even in Limbo.