The phrase "ammo crates" refers to boxes or containers that are used to store ammunition, which are bullets and other types of bullets used in firearms or weapons. Full definition
Power ammo crates now give more ammo for some weapon types and can now drop on player death if holding it. (twinfinite.net)
Once a player has accumulated enough money, they can begin to open doors to new areas, which will have other doors along with ammo crates, turrets, smart guns, ect. (mp1st.com)
If anything, it feels like there's a lot less choice in Killzone 3, but that's probably because there are so many ammo crates scattered around that changing weapons isn't a necessity. (gamingunion.net)