What is it? It says its reusable... do you stick it on the visor or something?

It's a small, soft cotton (?) cloth that's saturated with a waxy-feeling compound that comes in a rugged, reusable ziplock bag that's about 1.5" by 2.5" I've used the same one that came with a pair of long-gone Scott ski goggles for probably 15 or more years now. These days, it's tucked more or less permanently in a pocket on my moto jacket.

It works really well when you follow the directions -- after the lens (or visor) is fogged (I usually just breathe on it out in the cold before I start riding or skiing) you wipe the fog off with the cloth. It seems to stop fogging for a full day or two, but you have to repeat it more often than some of the other no-fog products.

The downside is that you have to use it more often than most products, but the upside is that it doesn't leave any streaking the way Cat Crap or any of the others do. I don't notice those streaks during the day, but at night the glare from oncoming headlights makes the streaks flare and it gets annoying.