Dimensional rifts are holes in reality, generally, this game is talking about gate rifts, which are Links between places that serve as a "Shortcut".
How to Make a Rift
There are three known ways to make a rift: Magic, Psionics and gravitational distortion.
All Rift Worlds contain at least three rifts, hence their name. These appear to be natural phenomenon, at least as natural as the worlds themselves...