I relate this picture to the feeling of being outside. You're watching people in the same room but you're not quite there with them.
I don't think It's the real meaning of the picture though. As you can see the woman/man on "the other side" of the pool is looking back att the boy. So I think it's more of a cool thought behind this. Someone clever whould have said;
"Which side of the picture is the real one?"