Sir please explain me about top 20 percentile for up board for passout 2019 student for apply in iit .

Look brother, it is very easy.

Top 20%ile in your X11 board indicates that you are among the top 20% of the meritious students who had appeared in your concerned board exams.

This percentile varies greatly depending on the nature of your boards. Generally state boards students at very percentage still achieves the top 20%ile status.

In our time CBSE students having above 86% (even I was also one of them) were the top 20%ile meritious students.

Failing in this criteria can lead to the elimination of that candidate from JoSAA counseling. Hence he won't be able to grab a seat at NIT/IIT/GFTI/IIIT.