What is headspace and timing?

"3-6. HEADSPACE AND TIMING Headspace is the distance between the face of the bolt and the base of the cartridge case, fully seated in the chamber. Timing is the adjustment of the gun so that firing takes place when the recoiling parts are in the correct position for firing. … Firing a weapon that has improperly set headspace and timing could result in damage to the machine gun, or injury to the gunner. Damage may also occur in the trunnion block, base of the barrel, or face of the bolt." Source: Field Manual 23-65, BROWNING MACHINE GUN CALIBER .50 HB, M2, HQDA, 19 June 1991. http://www.globalsecurity.org/military/library/policy/army/fm/23-65/Ch3.htm