Wikipedia
A semi or full-automatic firearm which is said to fire from a closed bolt is one where, when ready to fire, a round is in the chamber and the bolt and working parts are forward. When the trigger is pulled the firing pin or striker fires the round, the action is cycled by the energy of the shot sending the bolt to the rear which extracts and ejects the empty cartridge case, the bolt then goes forward feeding a fresh round from the magazine into the chamber, ready for the next shot.
When World War I era machine guns were being tried for use on aircraft, the Lewis gun was found not to be usable with a gun synchronizer for forward firing through the propeller, due to its firing cycle starting with an open bolt. The Maxim style arms used by both the Allies, as the Vickers machine gun and Central Powers, as both the rectangular-receiver lMG 08 and lightened-receiver LMG 08/15 Spandau gun, and Parabellum LMG 14 gun — as well as the improvements introduced by Swedish armaments designer Carl Gustave Swebilius to the American M1895 Colt–Browning machine gun for aircraft use, creating the M1917 and M1918 Marlin machine guns for the USAAS in World War I — all fired with a cycle starting with a closed bolt, and since the bullet firing from the gun started the firing cycle, it was much easier to set the synchronizer to only trigger the gun when the propeller's blade was not in front of the gun.