Sleep paralysis is basically when your logical mind is awake and your brain is taking in information from the real world, but the parts that control dreaming are still active. When you sleep your body paralyzes itself so you don't act out your dreams, so when you're in a sleep paralysis state, you're awake and can see and look around your room, but can't move because the body is still paralyzed, and since your brain is in sleep mode, you can get dream imagery or other odd hallucinations projected into your perception of the real world. Luckily I've never had the hallucinations, just the paralysis, which you can often fight off if you work hard to move.