A superjump is something that was purposely built into Halo2's maps by the Bungie staff that allows a player to exceed major heights after doing a certain glitch, something such as crouching down underneath a wall that allows the player to remain crouched without holding in the left thumbstick.
This trick/glitch/cheat is also completed by landing in a specific spot. The jump doesn't always go as planned. One may experience backfires such as:
Falling through the floor.
Being thrown in a different direction.
Getting stuck in the geometry of the level. IE: Ascension's Overshield railing.
The jump may not even take place, just a small bump up.
I'm gonna Superjump to get a better spot to snipe from.
Someone is on the roof, those bastards are cheating, superjumping.