Extracted from this article (see the bottom section):
Some roller-coasters have loop-the-loop sections, where you travel in a vertical circle, so that you're upside-down at the top. To do this without falling off the track, you have to be traveling at least a particular minimum speed at the top. The critical factor in determining whether you make it completely around is the normal force; if the track has to exert a downward normal force at the top of the track to keep you moving in a circle, you're fine, but if the normal force drops to zero you're in trouble.
I've been told the normal force is a contrapositive force. I mean, when I'm at the top of the loop, the weight already points down so how can there be a normal force, if I'm not exerting a force to the road?