A Man Cannon in Halo 3 and Halo: Reach, is a sorta short walkway that points upwards and shoots you across the level. Its often used to get to a different area quickly, but at times, has been used to fling objects to kill people. It can also be accessed by vehicle, to make for a very interesting turn of events. They are put in for a more strategic approach to combat, and exploration in a level. Its also a nice alternative to escape, while given a chance to take down a pursuer without taking a vehicle, or running away on foot.
In the now free downloadable level foundry, you can use the forge mode to pick up, delete, or set man cannon anywhere you please. This adds to the game as you cans set more than one, but there's still a set amount.