Hello Nikhil,

The vacant seats reserved for SC, ST and PwD candidates will be allotted to the candidates of respective categories for admission to a preparatory course of one-year duration on the basis of relaxed admission criteria (seeRank Lists).
Admission is given to the candidates in the preparatory course provided (i) the seats reserved for the respective category are vacant, (ii) candidates have not accepted a seat in the preparatory course earlier.
All the IITs will run preparatory courses of one year duration for SC, ST and PwD candidates intended to prepare them to pursue the academic program (for session 2017-18) to which they have been allocated. At the end of this course, candidates are declared as pass or fail.
On successful completion of the course, the candidates will be offered direct admission in July 2018 (Academic Year 2018-19) to the already allotted undergraduate course.
Candidates admitted to the preparatory course are also eligible to appear in JEE (Advanced) 2018 subject to fulfilling other eligibility criteria.
That is why you got a preparatory rank because your marks were not good enough for direct admission.

For more information about JEE Advanced, follow the below link:

Https://engineering.careers360.com/exams/jee-advanced