Unearth is an activated ability that works from your graveyard. Pay the unearth cost printed on the card — you can do this only as a sorcery, on your turn while the stack is empty — and the card leaves your graveyard and returns to the battlefield. The creature has haste, so it can attack or block right away. It is exiled at the beginning of the next end step, and it is also exiled if it would leave the battlefield for any other reason.
That gives a creature you have already used one more turn of work. A dead creature is normally finished, but unearth turns it into a final hasty attacker, a surprise blocker, or another round of an enter-the-battlefield trigger. Because the card is exiled instead of simply dying again, the return is strictly temporary, and opponents can plan around it — but they still have to deal with it.
