A hydrometer is a simple measuring tool that measures the density of liquids. Plain and simple water from the tap has a base density of zero. It is when you start adding sugar to the water, which is what the brewer is doing by steeping malted grains in water that the density changes. The more sugar present the higher the density of the liquid.
This hydrometer is used to measure how much sugar is in the beer before and after fermentation. The readings the hydrometer produce will enable you to measure the alcoholic content (% ABV) of your beer.
A hydrometer is also important to determine when fermentation is complete ensuring your beer isn’t bottled before it is ready.