Mock counselling is basically to give you an idea of which college you probably might get depending on the college preference of the students above you in the rankings.
What you are required to do in the mock counselling is just to fill top few colleges of your choice or priority of your choices of all the colleges that come under the exam on the portal. Now depending on the preferences of you and the students above you in the ranking and the total number of seats that can be allocated through that particular exam in a particular college, mock seats are allocated. And this is how you know, which college you probably will get depending on the students that have attended the mock counselling by filling out the portal.