Hop-up's main function is to increase the range of the BBs travel, making for longer shots. However, not all airsoft guns have a hop-up mechanism (check the make and model of your gun).
How it works
The hop-up mechanism on an airsoft gun imparts backspin on the BB. An adjustable piece of rubber provides resistance on the upper part of the BB when fired, causing backspin.
The hop-up lever adjusts the amount of backspin you impart on the BB -- from normal (zero hop) to full pressure. Fire a few shots to set the lever to its optimum setting. Start at zero hop, shoot a few rounds, adjust for more hop and observer the trajectory. If the BBs tend to drop, increase hop-up. When they begin to magically fly upwards, you're putting too much backspin -- lower the setting. Additionally, excessive pressure and backspin may also result in premature drop.
Since the hop-up mechanism imparts a spin on the ball, the spin is relative to the orientation of the gun. If the gun is fired sideways, the backspin is now turned into a sidespin (left or right, depending on how you tilt the gun), causing a hooked shot.