Behind the eight ball

Hi! Can you please tell me about this phrase? Thank you...

If you are behind the eight ball you are in trouble. In the pool game called
"eight ball" if you need to put the 5-ball in the pocket and the 8-ball is between
the 5-ball and the cue-ball, you have a difficult shot. You are not allowed to
use the 8-ball to hit the 5-ball.