I'd guess that it's probably a stroke with a glow of some kind added. But the important thing is that the camera moves have been tracked, so that the strokes can be added into the 3d space of the scene.
It seems to be replicating digitally something that stills photography can do in-camera. If you set a long and low exposure then you can draw in the air with a torch and have the drawing register (the only thing bright enough to be exposed is the light itself and the long exposure means you can move it around). What makes these shots more interesting is that you can then set off a flash which will bring the rest of the scene up to a registrable brightness but only for a fraction of second, freezing the action recorded. So far this might not make much sense; so here's an example of someone who probably did it first (Picasso):
The really interesting moving-picture application of this stills technique is in matrix-style time-slicing (I say matrix-style though Gondry did it earlier and others have claimed to have invented the technique too). I really wanted to do this myself, but some dutch guys beat me to it: