Crouch Jumping is a technique in which a player crouches directly after jumping. The move brings the player's legs up, reducing their height and allowing them to jump into higher places or smaller gaps. In Halo: Combat Evolved, it could also lower fall damage.
The technique has also been called "duck jumping", "high jumping", and even the "Gandhi jump".
The technique is extremely simple to perform. Jump, and crouch in midair. This should enable you to get to previously-unreachable places.
As previously mentioned, Crouch Jumping is useful for jumping to slightly higher locations or into smaller gaps. It is also an effective technique to use when strafing around an enemy during a firefight. It can be used to help teammates (in Co-op or team Multiplayer games) reach higher places.
The reason this trick works is not because it makes you jump higher. What actually happens is that, when you do a normal jump, the place you're trying to jump to is occasionally too high, and your feet will hit the platform edge, preventing you from landing on it. When you crouch jump, your hitbox becomes smaller (as it would while crouching), and the edge of the platform stops being an obstruction. Many Carney Holes require some form of crouch jumping, as does the Lonely Soul Easter Egg.
While mid-air, players can crouch and uncrouch repeatedly to gain more air time. Such a move can also, when carrying a one-handed weapon in Halo: Combat Evolved, make a player appear unarmed.
Those that use Halo 3's Toggle Crouch option may find crouch jumping more difficult, as the transition from a standing form to a crouching form is not as smooth with the setting enabled.