@donbeezy, it's a CO2 drop checker for monitoring CO2 level in my tank. I'm not sure how accurate it is, but it really helps me controlling the CO2 level.
It can only be used as a reference, pretty in accurate but gives you an estimate. The bad thing about it is that it takes a decent amount of time to change color. When something's gone wrong, such as an end of the tank dump, by the time the drop checker has changed color, your fish are almost dead.