Buyback is a keyword ability.
It is a static ability that appears on some instant and sorcery cards. As you cast a spell with buyback you may pay an additional cost. As the affected spell resolves, if this cost was paid, the spell will return to its owner's hand, rather than the graveyard.
Abraham is casting an Evincar's Justice, and want to buy it back. To do so he must pay in addition to the of casting the spell. After he does, and the spell has fully resolved, the card returns to his hand, rather than going to the graveyard as normal.
- 702.26a. Buyback appears on some instants and sorceries. It represents two static abilities that function while the spell is on the stack. "Buyback [cost]" means "You may pay an additional [cost] as you cast this spell" and "If the buyback cost was paid, put this spell into its owner's hand instead of into that player's graveyard as it resolves." Paying a spell's buyback cost follows the rules for paying additional costs in rules 601.2b and 601.2e-g.