doktadoom is right. Often a percentile is expressed as a single number, e.g. 58th percentile. If you're in the 58th percentile, it means you scored as well or better than 58% of the people taking the test. If you're percentile is a range, say 46-58%, it means you did better than 46%, as good or better than 58%, and 42% got a better score than you did.
Congratulations to all you MCATers. Good luck with the crazy year you have ahead of you.