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A semi or fully-automatic firearm is said to fire from an open bolt if, when ready to fire, the bolt and working parts are held to the rear. When the trigger is pulled the bolt goes forward, feeding a round from the magazine into the chamber and firing it. Like any other self-loading design without an external power supply, the action is cycled by the energy of the shot; this sends the bolt back to the rear, ejecting the empty cartridge case and preparing for the next shot.
Compared to a closed bolt design, open bolt weapons generally have fewer moving parts. The firing pin is usually part of the bolt, saving on manufacturing costs. Having an open chamber while not firing also helps keep the weapon cooler. However, accuracy suffers due to movement of heavy parts prior to firing, and lack of full lock up between bolt and barrel.
Open bolt weapons
Mixed mode weapons
Notes
In the U.S., the ATF made a ruling in 1982 that semi-automatic open bolt weapons are readily convertible to fully-automatic fire, therefore such weapons manufactured after the date of this ruling are classed and controlled as fully-automatic weapons (weapons manufactured prior to the ruling are grandfathered and are still considered semi-automatic).
See also
"Open bolt"