From The TimeSplitters Wiki

Health and Armor (especially Health) are vital in staying alive. If they both run out, the player dies. Health can be replenished with a Health Pack and armor can be replenished with a Armor Pickup.

Health

Health, as the name suggests, is the most important in surviving. In the games, it is marked by a red bar. This bar gets depleted as the player is attacked. If the bar goes empty, the player dies. The bar is replenished by Health Packs, which heal 10 or 90%, depending on the pack.
Health is depleted when the player is atacked or is on fire. Killing a player means comepletely depleting their health bar.

Armor

Armor is a sort of "second health". Unlike health, which is available from the start, armor must be found in the form of an Armor Pickup. When the player has armor and is on fire or being attacked, the armor must be depleted first before the player can lose health. Armor tends to go down quicker than health, especially when the player is on fire.
Like health, Armor is decipted as a bar. However, armor has a blue bar (health has a red one). The shorter this bar is, the less armor the player has.