Occasionally you’ll hear a brewer talk about the gravity of a beer and no, it doesn’t refer to Isaac Newton or the 2013 Sandra Bullock movie.
Simply put, a low gravity beer will have a lower ABV or alcohol content, and a high gravity beer will have more alcohol. But, more specifically, gravity is an indication of the relative density of the wort to water during fermentation. The larger the difference, the more alcohol your beer has.
When a beer is fermenting, yeast are busy converting sugar into alcohol and carbon dioxide. The decline in sugar and the increase in ethanol decrease the density of the wort. A brewer will take an original gravity reading at the beginning of fermentation and they’ll monitor the gravity over time. When the gravity stops declining, the brewer knows that the beer is ready and they will take a final gravity reading, which is often abbreviated as "FG."
In Europe, the original gravity is also known as Original Extract and when you hear someone talking about the “size” of the beer, that’s what they’re referring to; you may also see it printed on the label as a percentage or with the label Stammwürze. Finally, you may also see the gravity printed as “degrees Plato”, which is a European scale that indicates how much sugar is available to the yeast to convert to alcohol at the start of fermentation. So, the higher the degrees Plato, the higher the alcohol content.