The specific gravity of your beer determines how dense your wort is when compared to distilled water. Before your wort undergoes fermentation, it will have its highest reading of specific gravity. After fermentation is when it will be its lowest. Measuring your beer’s specific gravity before and after fermentation gives you two numbers (original gravity and final gravity) which can be used to calculate your beer’s alcohol content. During fermentation, yeast consumes sugars in the wort and produce alcohol while releasing carbon dioxide gas. This process causes your wort’s gravity to steadily decline and ultimately reach a stopping point indicating that fermentation is complete.
Measuring your beer’s specific gravity will require either a hydrometer or a refractometer.