It’s called the Schlieren effect and it means that you can see stuff that’s invisible to the human eye, like changes in air density. So when you turn on a hair dryer, you can see the blast of air it shoots out. When you open a can of Coke, you can see what’s escaping into the air. When you rub your hands, you can see the heat surrounding them.
Air is typically invisible to us (sometimes we can see the blurry heat on the road, for example), but it’s a constantly moving thing. Brusspup created a simple set-up—a camera with a razor blade in front of it, next to a light source pointing at a concave mirror so it bounces light back to the camera—to make it visible.
The trick in being able to see the invisible is that the razor blade (or any sharp edged object) partially blocks the light to create a shadow effect to reveal the air movement. I mean, it’s either that or sorcery.