The Shedmaster package is a free download on the PLL site. In really simple terms, Shedmaster acts like "Schedule at Trackmark".
A train driver can request that a new library schedule be assigned to the locomotive requesting it,
or
A train driver can request that a new library schedule be assigned to another locomotive waiting at a trackmark called an MPD (Motive Power Depot),
or
A specially modified industry can request that a new schedule be assigned to a locomotive waiting at an MPD.
Requests made by train drivers are done by means of a driver command called "Driver Communication".
That's the simple explanation.
Trevor