When a weapon fires from a closed bolt, it simply means that the bolt is closed when the trigger is pulled. Most civilian weapons function in this manner, same as the M2 Browning Machine Gun.
On the other hand, firing from the open bolt means that the bolt is locked to the rear, and as the trigger is pulled, the bolt rides forward, chambering the round and firing when closed. Upon releasing the trigger, the bolt will stop in the open position. Firing from a closed bolt normally results in better accuracy but an open bolt design will generally disperse heat faster.