This is a WAPI, a water pasteurization indicator. Basically what it does is it tells you when you have heated water hot enough and long enough for it to be safe to drink. Typically people believe that you need to boil water for two minutes before it is safe to drink, but the reality is that the entire time that the temperature was building up to boiling temperature, the Pasteurization process was in action.
 
Water will be fully Pasteurized and completely safe to drink long before it reaches the boiling point, but you need a device that can tell you exactly when it is ready; this is that device. Cheap, simple and effective. It belongs in every mess kit, survival kit or bug out bag.
 
This is one of the most valuable yet overlooked pieces of gear available; this is why I sell it, because it is fantastic and you need one!