Hi aspirant, 

Preparatory rank means you have not qualified JEE advance and have not got any rank in JEE advance merit list, so basically you cant take ny undergraduate coursenin IIT this year, but you may have a chance of securing a preparatory course(1 year)  in IIT and if you clear it you can have chance in undergraduate course in next academic year. However if you not interested in this you can have admission into NIT's witb you JEE main rank if you clear the cutoff. Hope you are clear with it.