Flying is an evasion ability. A creature with flying can't be blocked except by creatures that also have flying or by creatures with reach. If the only potential blockers on the other side lack flying or reach, your flier connects with the player, planeswalker, or battle it's attacking. Some cards pair flying with other abilities—flying and vigilance, for example—but the evasion rule stays the same.
Flying matters because it forces opponents to have specific answers. Ground creatures often sit uselessly on defense, while a single flier can chip away life totals or pressure planeswalkers turn after turn. It also changes combat math: a flying blocker can trade with an attacker that ground creatures couldn't touch, and reach lets a grounded creature join the defense. In deckbuilding, flying is a reliable way to push damage through cluttered boards.
