What does the 'hop" setting on an Airsoft gun do?
The term Hop, short for High Operation Power Up, describes the back-spin put on airsoft pellets and BB's to increase their range and (vertical) accuracy via Magnus effect. Hop-up acts somewhat like the rifling on the barrel of a firearm, but without the increase in horizontal accuracy. Also, being light-weight, airsoft pellets are still affected by wind when fired.
Airsoft hop-up devices apply a backspin to the pellet so that the pressure force acts on the pellet opposite to the direction that gravity is pulling it. This causes the pellet to fall less over a given distance than it would without the spin applied to it.