Hop-up is the backspin put on airsoft pellets and BBs to increase their range via the Magnus effect.
Hop-up devices apply backspin to the projectile reducing the air pressure on its top side. This causes the plastic pellet to fall less over a given distance than it would without the spin applied, extending the effective range of the weapon without increasing velocity.
In airsoft guns, this is usually implemented as a projection (called a bucking) into the path of the projectile, on the top of the barrel. The bucking momentarily hinders the top side of the projectile, imparting a backspin that can exceed 120,000 revolutions per minute. This is usually adjustable so that the effect can be tuned to suit the weight or speed of the projectile and each player's preference.