A hydrometer is used to figure out how much alcohol is in your beer or cider. It measures the density of liquid (called gravity). Alcohol is less dense than sugary pre-fermented beer, so measuring gravity at the beginning of fermentation (at the end of your brew day) and measuring it again two weeks later at the end of fermentation (when you're ready to bottle) will tell you exactly how much alcohol you have on hand. It's a great way to let a little science into your otherwise relaxing brew day adventures with a hydrometer.
Questions on how to use your hydrometer? See our ABV calculator.