First strike is a static ability that changes combat timing. A creature with first strike deals its combat damage before creatures without first strike do. That means if a first striker blocks or is blocked by an ordinary creature, the first striker hits first. If that damage is lethal, the other creature dies before it can hit back.
In play, first strike is about winning combat without trading. It lets smaller creatures beat bigger ones, or survive a block that would otherwise kill them. Granting first strike to another creature, as some effects do, can turn any attacker or blocker into a combat winner. It also works with protection, making a creature hard to block and hard to damage in return.
