With a closed bolt, the bolt and carrier are forward and locked, with a round in the chamber, and the hammer back until the trigger is pressed.
With an open system, the bolt and carrier are held back until the trigger is pressed. At that point, the bolt and carrier jump forward, strip off a cartridge, force it into the chamber, the bolt locks, and the gun fires.
Open is used on belt feds due to the great heat created by burning 50 or 100 rounds in one go. If a round was chambered soon after, it might "cook off".