The idea of parallel universes is certainly not something new. It has been entertained in popular media since the existence of universes existing simultaneously has been theorised in Quantum Physics. Many Sci-fi novels and T.V series scripts have been written based on the idea.
The basic assumption behind the notion is that there are possibly an infinite number of other universes besides our own which vary either in minor details or else entirely indistinguishable such as for example having completely different basic laws and constants of physics. Of course the most interesting possibility is the scenario of those parallel universes which might most closely resemble our own albeit with different personal and collective historic timelines.