I have had 4-5 major sleep paralysis episodes in my life and a bunch of little ones.
Basically it's when your mind awakes fully, but your body is still asleep. This either happens falling into sleep, or coming out of sleep, usually suddenly. Once, when I was little, my Mom called my name, it startled me awake, but I couldn't move or talk for a good 20 seconds before coming out of it. When I was little, I didn't understand what was happening or even that it was something unusual. I've read that if your normal sleep is disturbed or you wake up while your brain is switching between REM sleep to deep sleep it can trigger an episode. In my scariest episode later in life, I fully woke up but realized I couldn't move. I couldn't open my eyes, couldn't even make a peep. This happened multiple times across a 5 year period. The scariest one was I woke up, it sounded as if a freight train was in my ears, something in the room was so loud. (It turned out to be my box fan, my hearing was greatly affected) Then deep in my ears felt as if I had Bengay or Icy Hot in there. It was a sensation that you couldn't tell if it was hot, cold, or both. My tongue was pressed to the back of my throat causing a horrible gargly snoring sound. My whole body was completely paralyzed, so I couldn't even move my tongue. It was very scary. As I was coming out of it, the loud roar of the box fan faded to it's normal sound, the sensation in my ears slowly faded, and I slowly regained enough movement to move my tongue and breath normally. While this is happening, you almost get a euphoric feeling, at least I do. A lot of people only experience fear and cannot feel the slight euphoric feeling. As for me, I don't really have too much fear, so it feels very strange. One of my first major experiences, I was paralyzed with my eyes shut, but I felt as if I could see around the room in what looked like how they make echo-location look in movies. It reminded me of the movie Daredevil, where he can see with echo-location. I knew I wasn't though, so it must have been a hallucination. From what I've read, you can hallucinate while having a SP attack, but I've only had a hallucination once. As scary as they can be, I quite like the way it feels when nothing too scary is happening.