Degrees Brix is the sugar

Sugar

Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...

 content of an aqueous solution.

One degree Brix is 1 gram of sucrose

Sucrose

Sucrose is the organic compound commonly known as table sugar and sometimes called saccharose. A white, odorless, crystalline powder with a sweet taste, it is best known for its role in human nutrition. The molecule is a disaccharide composed of glucose and fructose with the molecular formula...

 in 100 grams of solution and represents the strength of the solution as percentage by weight (% w/w) (strictly speaking, by mass). If the solution contains dissolved solids other than pure sucrose, then the °Bx is only approximate the dissolved solid content. The °Bx is traditionally used in the wine, sugar, fruit juice, and honey industries.

The °Bx represents the same thing as the degree Plato (°P) widely used by the brewing industry, and degree Balling (the oldest of the three systems and still used in some parts of the world) found in older textbooks.

For example, a sucrose solution with an apparent specific gravity

Relative density

Relative density, or specific gravity, is the ratio of the density of a substance to the density of a given reference material. Specific gravity usually means relative density with respect to water...

 (20°/20°C) of 1.040 would be 9.99325 °Bx or 9.99359 °P while the representative sugar body, the International Commission for Uniform Methods of Sugar Analysis

International Commission for Uniform Methods of Sugar Analysis

The International Commission for Uniform Methods of Sugar Analysis is an international standards body, founded in 1897, that publishes detailed laboratory procedures for the analysis of sugar....

 (ICUMSA), in favor of "mass fraction", would report the solution strength as 9.99249%.