Percentile indicates how many students have been outperformed by you in exam.
Top 20 percentile means that you are ahead of 80% of the people.

To know more information you can refer below link

JEE Advanced top 20 percentile

Good Luck!!

Percentile indicates how many students have been outperformed by you in any examination as it is calculated like -

Percentile = no. of student have marks lesser than you / total no. of students appeared in the exam X 100

Your percentile should be >= 80 or you should have have your board percentage greater than or equal to 75%

Hope this helps!!!!