A stockpile is an collection of an item or items, typically in preparation for when that item becomes unavailable. A stockpile can be of anything but is usually of consumable items such as food, water, ammo, firearms, batteries, fuel, etc. Stockpiles are commonly kept in secure and possible secret locations since it's assumed that anything worth stockpiling will be worth stealing when the stockpile is needed. Stockpiles can belong to anybody, individuals, corporations, and private organizations; but most of the ones worth talking about belong to national governments. The USA and USSR both accumulated thousands of nuclear weapons during the Cold War and maintain significant surpluses long after. The biggest stockpile at present is the Strategic Petroleum Reserve in Texas which contains over a billion gallons of gasoline.
Stockpile can also be used as a verb. Stockpiling is the act of creating a stockpile and is a favorite pass time of Doomsday Preppers. Stockpiling is often unnerving when seen from the outside. It shows that one is worried enough to spend significant resources on future catastrophe. When governmental institutions do it it tends to set off alarm bells in the minds of conspiracy theorist which is kind of ironic given the heavy overlap of preppers and conspiracy theorists. I would make the final point of differentiating stockpiling from the more general term hoarding. One can hoard resources with the intention of creating artificial scarcity. Stockpiles are created out of a concern that one will actually be deprived of a resource if one doesn't hoard it.
IRON NODER: WE'LL RUST WHEN WE'RE DEAD