You might already know about general wait listing process in railways. It is represented by W/L followed by a number which represents your waitlist number. A W/L doesn’t grant you permission to board a train but a RAC does, which is the middle stage between W/L and a confirmed berth.
The above terminology applies if you booked a ticket in General quota. In case you booked your ticket under Tatkal quota and unfortunately get a waitlisted ticket, it will be indicated as CKWL followed by a number. Similar to General quota, RAC follows CKWL. A CKWL status after the chart is prepared bars you from boarding the train. In such cases, the passenger will get his ticket price back, minus few transactional charges.
Under tatkal quota CKWL is very common since the ticket purchase happens very fast, especially in busy routes (starting at 8 am). Even though you see tickets status as available, while proceeding to make your payment (online), your final ticket may show your booking status under CKWL, since there are only few tickets under tatkal quota and many passengers trying to grab them, all at the same time.
Although this varies from train to train, as a general rule, a CKWL up to 31 gets cleared (confirmed ticket).