What do Available, RAC, and Waitlist mean?

Available means there are still empty seats in the train and you will get a confirmed berth immediately after booking. RAC stands for Reservation Against Cancellation, which means you can board the train and the railway will allot you a shared seat usually a side lower berth that converts into two sitting berths. Waitlist means your ticket is not yet confirmed and you may not get a seat unless some booked passengers cancel before departure. If your waitlisted ticket does not confirm before the final chart preparation, it gets automatically cancelled and the refund is processed by IRCTC as per the railway refund rules.