Airlines typically sell more tickets than they have seats on a flight, knowing that some people will not show up. When too many show-up, the airline has to convince someone to take another flight (VDB - voluntary denied boarding) or IDB (involuntary - which can really upset the last person to check-in for the flight). So, they offer a voucher (usually a couple of hundred dollars) good for another ticket in exchange for you taking a later flight. Depending on the delay, airlines may offer meal vouchers and/or a hotel room for the night.

For more info., search for 'bump' or 'voucher'.