Counselling session means the process of Getting admission to college based upon Your qualification in the certain exam it is also referred to as seat allocation. For many courses in India, there is the common entrance exam for admission to different colleges so after entrance test you are allowed to fill choice with your priority and based on Number of applicants on a particular course and Your rank you will be offered admission. 

For Engineering there is common exam JEE Main which is responsible for admission in all the Engineering colleges of the country similarly we have NEET for Medical. There are also many other such exams.