There is a lot of bright light outside (from the Sun). This light reflects off the giant red monolith that just happens to be there. Again, some of that reflected light goes through the window so that the person can see it. Some of that light reflects off the window also. The person inside can see the monolith, but this light is too bright for the person to see the reflections from the inside of the room (but they are still there).If the inside light is not too bright, a person on the outside could not see inside because the light reflected from the outside is too bright. The light from the inside is still getting out, it is just too difficult to detect.This is essentially how a one-way glass works.