as swaty has correctly pointed out

minor area consists of a set of atleast 4 - 5 courses from a particular dept..

in iit's there are courses which you must be doing outside your dept if you do some of these courses in a particular deptt. you will get a minor area in that dept and it will be mentioned on your degree which you get

it's a very good option to realise your area of interest while stydying

as freshers a person has very limeted knowledge of a particular branch

once you have realised your interest you can do a minor area