Munitions are the things an army uses to fight: guns, bombs, bullets, rockets, and other military supplies. You hear it in news and official talk more than in the street — a soldier may say ammo for bullets, but a report will say munitions for the whole store of weapons and explosives. Though actually, English almost always uses it in the plural: we say munitions, not a munition, unless someone is being very technical. It can stretch a little to mean the stockpile, not only the weapons in someone's hands.