The Mother Board, or Mobo, or Main Board, can be described as the thing which everything attaches to. It is usually a plank of wood, which you attach all the boxes to. The originals were made of metal. A good example of a mother board can be found
here, and it is shown in Stefan's plans
here (click on motherboard).
The A.L.I.C.E. pack was used in the military to hold all their gear in. Some of these Alice packs had a metal frame. This frame, is attached to the “mother board” and is connected to your back. It is like a metal backpack, without the part that holds stuff. You can see the plans for a ALICE pack frame on Stefan's plans,
here (click on Alice Pack).
There you go! Anyother questions?