A staple of science fiction that is often used to explain how the characters are able to travel around the universe without having to go into a ton of crazy science.
While we have seen no visual evidence of a wormhole, it is basically a short-cut thru space and time. Imagine folding space and time over like a piece of paper, then creating a tunnel between the folded sheets. You could enter on one side of the paper and come out on the other without having to travel a great distance.
Want to know more about this interesting concept?
Jerry Coffey wrote about this at UniverseToday.com. Go check it out!