From The TimeSplitters Wiki

An armor pickup is an aid pack. It gives you body armor. Body armor is a like second health bar that protects your regular health bar. However it doesn't protect you from headshots, and is quickly depleted by fire. There are two different types of body armor. Armor pickups are the only way to gain armor, as unlike health, you don't spawn with any armor. In order to regain all of your body armor, you must get atleast two armor pickups.

Medium Body Armor

A Medium Armor Pickup gives you half of body armor bar.

High Body Armor

A High Armor Pickup that gives you 90% of your body armor bar.