Hello Shanker,

Top 20 percentile is a eligigbility criteria set for qualifying JEE where in you should be one of the top performers. 

For example: In a class of 1000, your score is 60%.

Lets say 400 people score below your percentage, then top 20% for yours will be calculated as - 400/1000*100= 40

Now your score is 60% but the top 20 percentile is 40

Hope this explaantion finds you well!