Browsing Nikon's website, I com across this product: AS-15 Sync Terminal Adapter. Product's description tells but I'm still confused. What is the general purpose of a Sync Terminal Adapter? Is it something to do with connecting the camera to the PC?
It's for cameras that do not have a PC sync port so that they're able to fire strobes that do have one, common in professional lighting equipment. So, in this realm, the PC means "Prontor/Compur" and that is (becoming "was") the standard port used by cameras to trigger off-camera strobes over a cabled connection. This has, slowly, started to be replaced by 3.5mm jack, when cabled, but it is still quite common and the port can be found on most professional and semi-professional cameras today.
However, for cameras that fall into the more amateur mode, the sync terminal adapter you linked to above is designed to provide the same feature via the hotshoe of the camera. Comes in handy from time to time, though less and less so as time goes by. I'd rather use radio triggering myself.
Wikipedia has a handy article on flash synchronization that has more detail.