Ghost fish hit a swung fly, but they don't get hooked. The ghost usually taps somewhere near the mid-point of the swing. Does the fish hit the nose of the fly and miss the hook? Or is it a sign that the fish is more curious than hungry? Hard to say, as I've also never seen a ghost fish. Sometimes the tap comes with a brief tug. But often, it's the equivalent of a gentle tap on the shoulder. By the time the signal is transmitted from the fly, up the leader, to the skagit line, through the running line to my hand the ghost is long gone. At times the tap is so subtle I second guess whether it was real or a figment of an overly optimistic imagination (or perhaps I had nodded off to sleep while standing waste deep in the run).
The ghost tap can be followed up by a real take on the next swing. More likely, the ghost tap simply serves to keep me in the run a little bit longer. Ghost fish are better than no fish.