If you are seeing a vase, then you are seeing the white area as the positive space. The black areas become the negative space. If you are seeing faces, then you are seeing the black areas as the positive space, and the white area as the negative space.
Love this explanation for positive and negative numbers. Positive numbers add a ballon, negative numbers add ballast. If you take away ballast (minus negative) the ballon goes up. If you add ballast (add a negative) you go down.