What is a Dimension?

In the Brandon Universe, there are many parallel dimensions that are accessed by ripping a hole in space and time or can be teleported to by Brandons. A dimension is a duplicate of the Brandon Universe, slightly or heavily modified. According to Einstein's theory of relativity, The Brandons need to reflect their light off the the edges of the universe, creating a super, weird, complicated quantum equation which can only be done by Brandons because they are the only beings that can harness that level of power. (except Gay Brandon)