Ammunition (ammo) is basically the same in the game as in real life. Simply defined, it is the amount of shots left in a given gun or the number of a certain item left (e.g. detpacks). This quantity is indicated by a number beside the picture of the weapon with which it corresponds. When your ammo on a certain weapon runs out, you are no longer able to use that gun until you pick up some more (shown in the picture) or stand next to an ammo droid. You will have to use another weapon if you cannot replenish your ammo- if all your weapons are out, you might as well respawn because you are pretty much helpless for the time being.