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

Also, you can go through the below links to check previous year NIT/IIT wise top 20 percentile -

https://josaa.nic.in/webinfocms/Handler/FileHandler.ashx?i=File&ii=86&iii=Y

https://josaa.nic.in/webinfocms/Handler/FileHandler.ashx?i=File&ii=87&iii=Y

Good Luck!!!