Hello Sahal

Percentile means how many students have been outperformed by you in the exam. Top 20 percentile means that you are ahead of 80% candidates.
The top 20 percentile is the marks scored by all the successful Candidates in their respective boards in the particular year. And so the candidates will have to score a minimum of 75% ( it is 65% for SC/ST), in their class 12 or be in the top 20 percentile of their respective boards to be considered for admissions.

How to calculate percentile !?
100 X  number of candidates in the group with aggregate marks less than candidate/ Total number of candidates in the group

Hope this helps.!
Good Luck.!