Indian Railways allows advance reservation typically up to 60 days in advance of the travel date. If you book early, there is a good chance of getting a confirmed berth. The coach and berth numbers will be indicated on the ticket itself. If confirmed berth is not available, you will get a ticket with RAC or WL status.
RAC stands for Reservation Against Cancellation, and it means that you don’t have a reserved berth, but you have sitting accommodation in the train and class of your choice. There is a possibility this will be converted to a berth when it becomes available due to last minute cancellations closer to travel date.
RAC ticket holders can board the train and are seated in the two side chairs. If a berth is available due to cancellation, then one RAC holder will get the berth, and the other RAC holder can convert the two side chairs into a berth.