Ammo
Ammo, short for ammunition, is broadly a term used to describe arrows for bows or bolts for crossbows. When it appears as an item property, usually on Quivers, it describes the number of remaining arrows or bolts held within out of the total capacity, which is the maximum number of those items that can be stored within. Since running out of ammo makes those weapons useless, it's important to keep an eye on the amount of ammo remaining.