The hydrometer is to the winemaker what the compass is to the mariner. The proper use of the hydrometer allows the winemaker to guide his activities precisely from pre-feremted must to final perfection and to discern all the steps in between.
Way back to 7000 BC, ancient civilizations discovered that any fruit juice (i.e. Grape juice) stored in a container began to ferment. When the process ended, the resulting juice was very different than the original juice. Wild yeasts, carried from the skins of the grapes to the juice during pressing, were the agents of change. Through careful observation he worked out a process that usually resulted in fermentation and, eventually, wine. Only in the recent past was the process truly understood, and with that understanding came tools to assist the winemaker. The most important of these, next to the fermentation trap (air lock) to avoid oxidization, is the hydrometer.
The hydrometer is a simple instrument that measures the gravity of a liquid in relation to the weight of water and it helps determine the amount of sugar dissolved in it. Hence the yeasts converts sugar into carbon dioxide and alcohol. By knowing how much sugar one started with and ended with, one can easily calculate the resulting alcohol.