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 at least two armor pickups.
In the first game, pickups gave a random amount of armor, from 10-100%, and their appearance differed depending on the time period. The earliest pickup was a metal breastplate, followed by a modern Kevlar vest, ending with a futuristic-looking vest with shoulder pads. This same series was used in TS2, and TSFP was limited to one type; a brown breastplate similar to that worn by the Crow's Kronia soldiers. The second and third games only have armor pickups that give either 50% or 100%.
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.