The Backpack is a large, green backpack filled with ammunition. The amount and type of ammo depends on the Backpack. For example, a Backpack found in one portion of a level may have 25 Shells, 200 Bullets and 10 Rockets, while one found later in a level may have 50 shells, 300 Bullets, 35 Rockets and 10 Sniper Rifle Bullets. The exception of this is Serious Sam Advance, where a Backpack gives the player 100% of every ammo type.
In the games it appears in, the Backpack usually appears before a big battle or during a wave of enemies in a large battle. When it's used in that way, picking up a Backpack usually triggers the next wave of enemies.
Backpacks are occasionally found in secrets, usually in the later portion of a game.
If a Backpack does not appear before or during waves of enemies, try to avoid picking up a Backpack unless one's ammo is low. Picking up a Backpack when the player has a lot of ammo is a waste of ammo in the Backpack.
Backpacks found in secrets usually have a lot of ammo in them, so the player should try to seek out secrets later in the game if they're having some ammo problems. A Backpack found in a secret can make a huge difference.