Percentile means the total number of candidates are behind you .Top 20 percentile means that you have scored 80 percentile in an exam which means your rank is 80% above than the total candidates appeared for the exam
Here is the link for more detailed information : https://engineering.careers360.com/articles/top-20-percentile-cutoff-of-class-12-boards-for-iit-admission
Regular exam updates, QnA, Predictors, College Applications & E-books now on your Mobile