RAC stands for "Reservation against Cancellation". It is an intermediate status between "Waitlist" and "Confirmed Ticket". With Waitlist, you cannot board the train at all.
With RAC, you get "half berth". You share a berth with another RAC passenger, and can board the train to travel. While the train is en route, if the TTE finds that some people did not board at the last minute, he will allot you the "full berth" according to your queue status. If not, then you travel the whole journey sharing the berth.
But, usually, RACs advance...
more... to full confirmation - but there's no guarantee. It's your choice whether you want to travel.