Hey pakhi

Percentile means relative score.

It is base upon the total number of students and number of student who score more than you.

Example: if in a test of 20 marks 10 students appear.and their marks as follow: 

2;4;6;7;9;10;12;13;14;15

Than student who score 15 marks have 100 percentile and similarly student who secure 12 marks have 70 percentile.

Formula= 100* ( number of students score equal or less than you)/ total number of students appear

Good luck