Reach is a keyword ability that lets a creature block creatures with flying. A creature with reach doesn't have flying itself, but it can still intercept fliers as though it did. That means an attacking creature with flying can be blocked by a reach creature the defending player controls, so the air isn't automatically safe.
In play, reach is a defensive tool. It lets decks without flying — especially green decks, where reach shows up most — answer evasive attackers and shield themselves from damage. Creatures with reach often have reasonable stats for their cost, so they can hold the ground while still contributing to combat.
