The "Man Cannon" tosses a player through the air to another part of the map. The user is said to be unprotected while in mid-air, but can still attack other players. It is much like the Covenant's Gravity Lift technology, possibly the Covenant learned this from the Forerunners. The Man Cannon works as a kind of replacement for the teleporter, but the teleporter may still exist in other maps not yet shown.
While the name specifies a man, the cannon will launch anything that enters it. Unfortunately, its strength will be diminished on heavy objects such as Warthogs, and not launch them as far.[1]
As of now, this device has only been featured in the Halo 3 Multiplayer map Valhalla, but a similar device can be found on the Halo 2 Multiplayer Map, Ascension. As shown in the new Halo 3 Vidoc, Bungie created the Man Cannon to replace the teleporters because it adds some challenge and risk to the gameplay. For example, a player may be sniped while in mid-air. It also adds some advantage, as this player still can attack the other competitors.