From Halopedia, the Halo wiki
A collision hull, also known as hitbox or collision model, is an invisible 3D model placed around an object in a video game. This model is used for collision detection, and is sometimes referred to as a collision geometry file. The name "hitbox" is derived from older games, which used a simple cube.
The visible models of objects (such as Spartans and Sangheili) tend to be very complex, possessing large amounts of surfaces and model geometry. If collision detection was used on these models, it would be extremely inefficient. Collision hulls solve this problem; they are simplified versions of the objects they are used for. A collision hull for a humanoid figure, for example, may resemble stick figures, particularly those commonly used on public signs. A collision hull for a stairway may be a single, flat surface. Vehicles also have collision hulls; the game engine recognizes these as physics tags and collision tags, and uses them to determine how a collision between a vehicle and another object will occur.
Because collision hulls are simplified, they do not always match up perfectly with the objects they are connected to. In some cases, part of a collision hull may extend past the invisible object, giving the appearance that one can shoot the object by shooting the air next to it. In the case of a vehicle, this would cause a collision to occur with no visible point of contact. This is most notable in Halo: Combat Evolved, as the vehicles used simplistic collision hulls. In other cases, parts of the visible model may not be contained within the collision hull, so that bullets pass through parts of the object as if the object was not even there.
The hit box is the reason that when standing on the edge of a wall, your character will "float"; the hit box is still resting on the wall.
It was once thought that Armor Permutations affected a player's hitbox; the contrary has been proven and stated by Bungie in numerous Bungie.net Weekly Updates. Sangheili and Spartan IIs use the same hitbox in Multiplayer, explaining the Sangheili Neck Glitch.