A Dimensional Door is an item in the Dimensional Doors mod. Uses
The Dimensional Doors mod adds Pocket Dimensions , and Dimensional Doors are a way to get to them. Each Pocket Dimension is a certain number of steps away from the overworld. Stepping through a Dimensional Door in the overworld takes you to a Pocket Dimension one step away from the overworld. You can then place a Dimensional Door in that dimension. Going through a Dimensional Door takes you one more step away from the overworld.
Warp Doorwill take you one step in the other direction, and a will take you straight to the overworld.