Tom Cruise's character was in a bad accident. He turned into a miserable recluse who was always in pain. So instead of living his life in pain, he commited suicide and lived in something like a dream like state, except he was dead, they called it a 'lucid dream'. In the end, when he figured out that he was really dead and it was all just a bad dream, they told him he could go back and get un-frozen (it was 150 years later) or he could continue with the dream. He chose to be un-frozen and live a real life.
I actually really enjoyed this movie, it took a little thought to keep up but once I figured it out, I was very into it. On the other hand, I got DH to watch it with me the other night and he hated it.