What is sleep paralysis and it's causes?

I remember when i first had SP. for me it wasnt to bad i knew wat it was so it helped. i just woke up in the middle of the night and i was laying there and i couldnt move. i was wondering when i realized wat it was and went with it. i started to hear someone whispering but wasnt afraid. then i heard hard rain but was ok because i like the rain then it just went away soon. i didnt see anything and i think i was on my back which i usually dont sleep on.