Bump Possession allows players to control what would normally be an NPC. In other words, in Campaign, the player would have the ability to play as Marines, Flood Combat forms, Sentinels, and essentially anything with AI; this can sometimes just be done by touching the character.
When a player controls an NPC, all of the NPC's allies will become the player's allies (so it wouldn't make sense to control a Covenant Elite and be fired upon by your own troops as if you were still the Master Chief).
If you possess an elite, such as one from the first Spirit dropship on the level Halo, and have the grunts alive, then the grunts will follow you around as if you were the squad leader of the grunts
warning:if your character dies and you plan on completing the level dont let him die because hitting a checkpoint while controlling a character which is not yourself makes it so you cant move at all..