A fabric storage pocket.
Bags are wonderful and all, but they are easy to steal, and getting things in and out of them takes up valuable time. Pockets sewn onto clothing alleviate this problem by literally allowing the wearer to wear bags on themselves. Pockets are most commonly used to store valuables such as cell phones, car keys, and wallets; items that may come in handy, such as tissues and handkerchiefs; and items that need to be removed quickly, such as firearms and knives. Most pockets in civilian clothing cannot be closed, or are closed with only a simple flap. Some pockets may have a zipper or button to close them, preventing the contents inside from falling out.
Usually found on military and law enforcement uniforms to store ammunition.