This is when you lightly hold your gloved hand over the rolling tire in order to remove any little shards of glass or metal it might have picked up when you rolled thru that patch of loose crap at the last intersection. The front tire is dusted with your hand just in front of the fork crown. On the rear, you dust the tire between the seat stays and seat tube. Just make sure your hand doesn't get caught between the tire and the seat tube; it CAN happen! Hold your hand close to the seat stays.
You only need to do this for smooth, thin tires. You've probably already seen this done by experienced riders and just didn't know what it was called.