The term Hop-up, short for High Operation Power Up, describes the back-spin put on airsoft pellets and BBs 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 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.
In airsoft guns this is often implemented as a rubber piece at the rear of the barrel that is thicker at the top of the barrel than the bottom. As the pellet moves past this piece it tends to roll, inducing a backspin.
This is most often adjustable so that the effect can be tuned to suit the weight or speed of the pellet and each player's preference.