RAC means reservation against cancellation it does not provide you full berth, it actually provides half seat so this means two people will be sitting on one berth. There is a chance of getting the seats confirmed. It is basically the buffer between the waiting list and confirm seats so gnwl 2/rac 7 means at the time of booking the waiting list was 2 and now the current situation is RAC 7. During the chart preparation you can also see the coach number. RAC is a way to compensate between the seats that are left by many people having confirm berth or travel for short distances in long distance trains.