Degrees Brix, written °Bx, express concentration on a scale calibrated to sucrose in water. In a pure sucrose solution, 1°Bx corresponds to 1 gram of sucrose in 100 grams of solution. Refractometers commonly estimate it from refractive index.
In fruit juice and other foods, acids and other dissolved solids affect the reading, so Brix is not always an exact sugar percentage. It is useful for consistency checks in syrups and preserves, but it is not a stand-alone food-safety test.
Reference: Further reading.