If you do not check-in on time, the airline is permitted to cancel your reservation, even if confirmed, and you will lose your seat on that flight. Check- in times vary from carrier to carrier, and are found in the carrier's tariff. To illustrate, if an air carrier has sold too many tickets for a flight, it may begin to deny boarding to certain passengers. Quite often, the first people to be denied boarding are those who have missed the established check-in time and no longer hold confirmed reservations. In most, but not necessarily all cases, your airfare is guaranteed once your airline ticket has been issued and paid in full.
Carriers often sell more seats on an aircraft than are actually available.
Carriers have extensive data that allow them to plan for the numbers of no-shows for a particular flight. However, on any given flight, more passengers may present themselves than there are seats available. Carriers have established rules for the treatment of bumped passengers. These can be found in the air carrier's tariff. In most instances, carriers ask for volunteers to give up their seats for some compensation.