Warp Door is one of the doors added by Dimensional Doors.
There are three methods to crafting the Warp Door. The first is the most expensive method, using up an entire Ender Pearl for one door. The alternative takes a bit more time and effort, using Stable Fabric instead. The third, however, actually uses nothing but a single wooden door and a pre-generated rift in the world. Putting a door on the same block as the Rift will turn it into a door to that Pocket Dimension. The same works for Iron Doors.
Warp Doors have two main uses. Each Pocket Dimension is a certain number of steps away from the Overworld. Traveling through a Warp Door will take you one step closer to the Overworld.
A Warp Door used in the Overworld can be used for teleportation. Use a Rift Signature to mark the two places where you want the doors, then place a Warp Door over one of them. Going through the Warp Door should instantly transport you to the other location.
Technical Block Edit
Warp Door is also the name for a technical block for Warp Doors added by the same mod. They are only used when a Warp Door is placed and acts as the top half of the placed door.