Hey.

Top 20 percentile would indicate that you hace outperformed or you are ahead of the rest 80% of the people who also took the respective qualifying exam.

 The top 20 percentile of class 12 boards will be considered for admission to the 31 NITs, 23 IIITs and 23 GFTIs. Candidates have to fulfill any of the two conditions for admission –

(a) score minimum 75% marks, or

(b) be ranked in the top 20 percentile in their respective boards.

Due to different scoring pattern for each syate, the cutoff marks for the top 20 percentile candidates varies across boards. 

You can use the linl below for any other details.

https://www.google.co.in/amp/s/engineering.careers360.com/articles/top-20-percentile-cutoff-of-class-12-boards-for-iit-admission%3famp