Hii

In JEE Mains, the percentile score is calculated by taking into consideration, the total students who appeared, and the no. of students who scored marks equal or less than you.
The percentile is calculated by following formula -
100 * no of students of that session who have score equal or less than your score / total no. of students in that session
So coming to what 51 percentile means.
This means that 51% of the students who appeared in your session have scored marks equal or less than your marks and that only 49% students have marks more than you.
In simple words you stand ahead of 51% of the students.

you can refer to following link for more details
https://engineering.careers360.com/articles/jee-main-normalisation-process-how-scores-are-calculated

Hope this helps
Thanks!!