Chandler's "The Campaigns of Napoleon" places fairly heavy emphasis on the significance of "lines of communication" for Napoleon and his opponents; what exactly were these methods of communication, and how were they established/secured in a way that made them more efficient than a typical courier system for dispatching orders?
There's some confusion here: lines of communication mostly means "supply routes" rather than literal communications (though that is certainly important). Napoleon, through an extensive system of alliances and treaties in Europe, secured food and ammunition for his armies despite the vast distances involved. Remember, this is all before railways!
The term "communication links" is still used in geographical disciplines to refer to road, rail, water and air links specifically in reference to the goods carrying capacity. One can only imagine the amount of work it took to support an army in the Russian winter across unmarked roads in a horse and cart... and quite how bad it must had been when that collapsed.
To put it at its absolute simplest the way communication, supplies, and therefore usually armies go from one area to another.
This can be from the capitol to the army, it can be from a fort to an army, it can be any number of specific things, but at its barest it's a simple case of the path communication can and must follow from A to B.