It is a static ability that creates an additional combat damage step. A creature with first strike will deal its combat damage before a creature that doesn't. This often leads to the other creature dying before it gets a chance to strike.
Allen attacks with a Mindsparker, which Nate then blocks with a Canyon Minotaur. Normally the two creatures would kill each other as they have enough power to deal lethal damage. However, because Mindsparker has first strike, it will deal its damage first, which will kill the Minotaur before it deals its damage.
702.7. First Strike
- 702.7a. First strike is a static ability that modifies the rules for the combat damage step.
- 702.7b. If at least one attacking or blocking creature has first strike or double strike as the combat damage step begins, the only creatures that assign combat damage in that step are those with first strike or double strike. After that step, instead of proceeding to the end of combat step, the phase gets a second combat damage step. The only creatures that assign combat damage in that step are the remaining attackers and blockers that had neither first strike nor double strike as the first combat damage step began, as well as the remaining attackers and blockers that currently have double strike. After that step, the phase proceeds to the end of combat step.
- 702.7c. Giving first strike to a creature without it after combat damage has already been dealt in the first combat damage step won't prevent that creature from assigning combat damage in the second combat damage step. Removing first strike from a creature after it has already dealt combat damage in the first combat damage step won't allow it to also assign combat damage in the second combat damage step (unless the creature has double strike).
- 702.7d. Multiple instances of first strike on the same creature are redundant.
First Strike • Flash • Flying • Haste • Hexproof • Indestructible
Lifelink • Menace • Prowess • Reach • Trample • Vigilance