Buyback is a keyword first used in Tempest block, which was released just under 14 years ago. It is similar to other keyword mechanics, in that it allows you to get some additional use out of the instants and sorceries in your deck.

Technically speaking, buyback is two abilities. The first is one that allows you to pay the buyback cost when casting the card. The second ability generates a replacement effect that will return the card to your hand when it resolves, as opposed to going to the graveyard.