On the dimly lit side of the glass very little light can make it through the glass so people on the other side do not have enough light waves to see. There also are not enough light waves relfecting for people on the dimly lit side to see their own reflections.

On the brightly lit side since some of the light is reflected, the people on this side can see their own reflection. People on the other side can see the light that was transmitted through the glass so they can also see what is happening on the brightly lit side of the mirror. It just looks like they are looking through dark colored glass.