The graveyard is the zone where discarded, destroyed, sacrificed, milled, and otherwise spent cards usually go. Many Magic cards treat it as an active resource, allowing cards there to be replayed, returned, counted, exiled, or used to fuel other effects.
Muldrotha, the Gravetide can replay several permanent types from the graveyard, Crucible of Worlds lets lands be played from it, and Living Death can turn a stocked graveyard into an entirely new battlefield.