What is a wormhole in physics?

I believe that the idea behind a wormhole is that one could use them to travel between two points in the universe which are considerably far apart in distance in a fraction of the time it would take to traverse said distance conventionally. I think they are theorized to be a burrow through subspace, thus serving as a shortcut to destinations which are far away. I also believe that they do not exist, except for in science fiction.