A lighting and photography term. "The subject is in open shadow" means that the subject is not being hit by direct sunlight, but that the light from the sky falls on him.
open shadow is a "soft" light, not quite as soft as overcast lighting. As an indication, I would say that it is a good light to take simple B/W portraits. For color work, you should remember that it is a severely blue light, which will lead to heavy color shift.
You should also watch out for light that has been filtered through leaves or that bounced on grass: green is not a very comely hue for the human skin.