Body armor is one of several special object types that extends the normal health meter. The amount of armor can be set for each collectible, although the typical amount is equivalent to a full health bar.
If the armor is shot or destroyed, it can no longer be picked up.
Some of the multiplayer community frowns on body armor. The Dr. No cheat was created to remove body armor placements in all stages through use of a toggle in the in-game cheat menu. It is only available through hacking.