The ZU-23-2, also known as ZU-23, is a Soviet towed 23 mm anti-aircraft twin-barreled autocannon

Autocannon

An autocannon or automatic cannon is a rapid-fire projectile weapon firing a shell as opposed to the bullet fired by a machine gun. Autocannons often have a larger caliber than a machine gun . Usually, autocannons are smaller than a field gun or other artillery, and are mechanically loaded for a...

. ZU stands for Zenitnaya Ustanovka (Russian: Зенитная Установка) - anti-aircraft mount.
ZU-23-2 was developed in the late 1950s. It was designed to engage low-flying targets at a range of 2.5 km as well as armoured vehicles at a range of 2 km and for direct defense of troops and strategic locations against air assault usually conducted by helicopters and low-flying airplanes.