A gas-operated firearm is a type of auto-loading firearm which harnesses some of the pressurized gas produced by discharging the firearm to actuate the mechanism of ejecting the spent case and loading the next cartridge.
A small gas port is located somewhere along the length of the barrel. After the bullet passes the gas port, some of the pressurized gas can escape through this port, which then applies pressure to a piston or bolt carrier, setting the auto-loading mechanism into action.