Its when the goal isn't to have the fish see the lure and bite it, but rather to get the leader into the fishes mouth like dental floss. Hopefully the hook will wind up embeded in the fishes mouth somehow, whether through its own movement, your detection and strike, having the line caught in its teeth, or a combination of all of the above. It's not really as sporting or exciting as actually having fish take the bait, but some fisheries are mostly a floss fishery.

A prime example of this type of fishing is Jim Teeny sight fishing for salmon or steelhead in riffles and flossing them with his nymphs. There's a real skill to it - you just have to decide if its the kind of skill you want to spend much time developing. When its the only way to catch fish because they simply aren't biting, what else do you do? Either stay home or floss.