POTH is an incredibly useful tool for both testing and routine observing. It started out as a "plain old telescope handset" (hence the acronym), but it has grown a lot since then. We can thank Jon Brewster for POTH. Let's look at a block diagram of a typical usage:
MaxIm DL's telescope controls are fed into POTH, which calculates the proper dome slit azimuth for the mount geometry. POTH passes commands from connected programs through to the drivers, allowing the programs to share the devices. Both MaxIm DL and Cartes du Ciel are controlling the telescdope and reading back its coordinates, etc. A slew from either will cause the dome to move to the proper new slit azimuth. Also, POTH will not declare either the telescope or the dome to be finished moving until both have stopped moving. Thus, MaxIm's imaging sequencer will not start an exposure until everything is settled. In the diagram, FocusMax could be attached to the focuser and telescope (for AcquireStar) as well.
POTH has many useful features such as traffic logging and its own set of "handset" controls for the telescope, dome, and focuser. There is a detailed help file for POTH, accessible from the Start menu, ASCOM Platform, Scope/Dome hubs. It also has a "quiet" mode which minimizes traffic to and from the driver, useful for ill-behaved driver/controller combinations that might have timing issues. In quiet mode, POTH will not respond to changes made using the telescope's hardware handbox.