Hello Jessypal,
The preparatory ranks are given to you on the basis of the number of vacant seats left under your category reservation.That means, in your case , if there are vacant seats not yet taken by SC category candidates,they give it to next set of eligible candidates from the same category and train them for regular admission. 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. Through this rank you can get admission into one of the preparatory course which is 1 year long at the end of which you will be declared pass or fail, followed by a seat in regular undergraduate degree.

Hope this helps
All the best.