What does it mean when they say my baby is in the 15th percentile?

My child was in the 50th percentile and now in the 15th. My doc is putting me on 2 NST's a week not until the baby is born. I am epileptic and not sure if that has anything to do with the weight of my baby although I will be asking when I go in Thursday to c her. But besides the weight of the baby what does the 15th percentile really mean and is there any website I can go to to read up on it and should I really be worried?? Thanks!!!