Airsoft guns shoot a perfectly round 6mm plastic BB. This BB, if it came out of the barrel without any rotation, would knuckleball and not have any semblance of accuracy. A traditional rifle style twist would stabilize the BB somewhat, but the BB would not go any farther than the knuckleball style BB with no spin. Airsoft BBs will fly straight and farther when a backspin is put on the BB.
This backspin has a name, the Magnus Effect. If the spin imparted to the BB is too great, the BB will curve upward rapidly. When tuned in just right, the BB will fly straight and for a greater distance than it would otherwise.
Many airsoft guns will have an assembly at the breech end of the barrel that employs a section of rubber, called the bucking, over an opening in the top of the barrel. A collar piece rides over the barrel and bucking. This piece is thin on one end of the adjustment range and thick on the other. By turning this collar the level of pressure placed on the bucking changes.
Once you select the weight of BB you want to use with the airsoft gun, you can adjust the hop-up so that the BB flies best.