stats

It is what percentile you rank among the pool of players in the rating pool; i.e. your rating is better than that percentage of players in that pool.

If you have a rating of 1200 on chess.com blitz/standard/bullet and the website gives you a corresponding percentage of 50%, this means you are in the 50th percentile of chess.com's rating pool.  This tells you that you have a dead average rating.  Geting ranked in the 10th percentile (10%) means that you perform in a lower group with a smaller rating.   Geting ranked in the 90th percentile (10%) means that you perform in a higher group with a larger rating.  

Think of percentiles like this.  They are 100 percentiles meaning 100 different groups.  These groups are ranked from 1 to 100 from smallest to largest.  Many people can be ranked together in a single group.

That's better than than my not-so-accurate explanation 

Thanks. That's sort of what I thought,