From Halopedia, the Halo wiki
Wraith Jumping is a type of Super Jumping used in Halo 2 and Halo 3 which involves having another player boost a Wraith into you in an attempt to propel yourself higher and further forward than would be possible with an average jump. Although Wraiths are not always readily available, Wraith Jumping can be more effective than other super jumping techniques such as using Frag Grenades due to the great impact of the Wraith. However players attempting Wraith Jumping often use Overshields or similar protection to avoid being killed.
Have a player find a wall (must be accessible by a Wraith) and have that person walk backwards, against the straight wall, this player must have an Overshield. Then, get a second player to get into a Wraith and use the boost to ram into the player on the wall. The player will be boosted into the air at a great speed and obtain a great height, being able to access hidden parts of the map or escape the level boundaries. If done in multiplayer, a player may get near the boundaries and may be killed by the The Guardians.