From The TimeSplitters Wiki

Armor Pickup

An armor pickup is an aid pack. It gives you body armor. Body armor is a second health bar that protects your health bar. But it doesn't protect you from headshots. There are two differnt types of body armor. In order to regain your body armor, you must get another armor pickup.

Medium Body Armor

A medium armor pickup gives you half of body armor.

High Body Armor

A High armor pickup that gives you full body armor.