From Halopedia, the Halo wiki
A Slide Jump is a trick that utilizes a combination of jumping and crouching to gain height and distance by using multiple jumps. The naming comes from the 'slide' that occurs when landing the slide jump.
The slide jump is one of the most basic types of trick jumping in Halo and is fairly easy to do, although it takes some practice before mastering.
The first step, as with any jump, is to actually jump. While you are in midair, press and hold the crouch button until you land on the area you choose to land on. Immediately after you touch the surface, release the crouch and press the jump button to enter another jump.
The purpose of the slide jump is to continually bounce off surfaces until you reach the desired destination of your choice.