Rim lighting...is light around the rim or outline of your subject. I'd suggest that the main purpose that it serves, is to give your subject separation from the background...however, it can also give your subject (especially people) a glowing quality...even a halo effect.
It is created by light that comes from behind your subject. Of course, it won't show up if your subject is doesn't have some sort of contour...but thankfully people aren't two dimensional.
You see rim lighting in TV & movies all the time....especially when the actors have big/bushy hair. They are lit as if the sun is behind them. Often, several actors in a scene...facing different directions, will all have back/rim lighting...which isn't natural looking...but it looks good on screen.