Question about lightyears?

No, it means that it would take light 20 years to reach. Example: If another galaxy is 20,000 light years away, that means the light from that galaxy took 20,000 years to reach our eyes. So we're seeing the galaxy as it was 20,000 years ago. That's why looking up at the stars is like looking back in time of how the universe looked thousand of years ago. A wormhole is a distortion in space and time. Say if you take a piece of paper that represents the universe and fold it in half, a wormhole is the place where the two sides would touch and if you were able to travel through it, you would end up in another part of the universe. It's just a theory though, there is no evidence that it exist outside of Star Trek that is.