Light fall is sitting on the bike at a stop light and your foot comes down on empty air instead of pavement because of a pot hole you didn't see. Your foot eventually gets to the ground, but you are past the point of no return on the bike tipping and you use all of your strength (and pulled groin muscles) to slowly lower the bike to the ground.
I've had some bad falls over the years. When you see your 929 flipping end over end while your sliding down the highway, that's a hard fall. A light fall is a 5-10 mph low side from some sand. So one light. One hard.