The single action of a firearm refers to its trigger that performs only as a single action. Where the only action is firing the weapon, the gun is cocked manually by hand. If the trigger is pulled when the hammer is down, it will have no effect. The most popular single action revolver is the Single Action Army, or SAA, seen on almost every gun-toting character in Wild West movies.
There are also single action semi-autos, which, like single action revolvers, are also only used for firing the gun. The mechanism must be manually cocked to fire the first round, the reaming are loaded using the kinetic energy from the recoil to eject the empty cartridge and load the next one into the chamber and re-cocking the firearm. This makes it instantly ready to fire the next shot.