What does parallel universe mean?
A parallel universe refers to another universe which co-exists independently of our own.
The concept of parallel universes is theoretical. Parallel universes are a popular feature for science fiction writers.
The idea is that our universe isn't the only one. There are others which co-exist with our universe. If they exist they are independent of our universe and we don't know whether its is possible to travel between them.
One feature of our universe is that the physical constants are very finely tuned. If any of the constants had a slightly different value then the universe as we know it wouldn't exist.
Some theories about parallel universes are that they have different values for physical constants. There would be some universes where stars can't form for example. Other theories have a faster speed of light or different geometries. The hyperspace of science fiction is a parallel universe which allows for faster than light travel between two entry points in our universe.
Another concept involves what happens when an event occurs. If there is a decision point which say two options, the theory suggests that the universe splits into two at that point each universe having a different outcome of the decision.
There are also theories that black holes are portals to parallel universes.