That should keep you busy for a few years!
A simple description is that due to the large number of merchant ships being sunk by the German submarines, the government installed limited numbers of guns on merchant ships, and provided a small Navy Armed Guard crew to man those guns. In addition to gunnery personnel, there were often signalmen who were responsible for communication between ships using flags, semaphores, blinker lights, etc. There were also sometimes radio personnel as part of the Armed Guard crew. The Armed Guard men were part of the US Navy, and not Merchant Marines. They served alongside the merchant marine crews who handled all other aspects of the merchant ships. Armed Guards also served on troop transport ships which were operated by the Army and the Navy.
The Armed merchant ships usually sailed in convoys with a small number of Navy ships as escort.
During the war, more than 140,000 men served in the Armed Guard.
Hope that helps.