Flying is a creature only ability that makes it so the creature can only be blocked by other creatures with Flying, Reach, or have an ability that says they can block flying creatures.
Essentially, flying allows the creature to hit the player directly if nothing else on the opponents side has flying or has the ability to block flying. However, there are also disadvantages to have flying as an ability. Certain cards damage specifically flying, or even do DOUBLE damage to flying creatures.
Flying has an very old equivalent called Horsemanship, which was secluded to only 1 set.
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