Looks like younger brother Andy performed the song but the older three wrote it. Seems like a dream about a girl. Beyond that my pea brain cannot grasp.
Actually it wasn't the Bee Gees, it was Andy Gibb. Andy was the youngest Gibb brother but not a member of the Bee Gees. But Barry, Maurice and Robin did help write Shadow Dancing and may have been "back-up" when it was recorded.