Triple scale hydrometer. This hydrometer is a glass bulb that is weighted and floats in a testing jar. The hydrometer will tell you approximately how much alcohol was made in the fermentation process. The scale on the Mile Hi Distilling hydrometers is called a triple scale which gives you three scales. A brix reading, a specific gravity reading, and an approximate potential alcohol reading. Hydrometers will not work in a distilled spirit.

See how to use a hydrometer article here 

If you’re more of a visual learner you can watch a video on how to use a hydrometer  here