What is 'the brix'?
Degrees Brix (symbol °Bx) is the sugar content of an aqueous solution. One degree Brix is 1 gram of sucrose in 100 grams of solution and represents the strength of the solution as percentage by mass. ... The °Bx is traditionally used in the wine, sugar, carbonated beverage, fruit juice, maple syrup and honey industries.
Short story, it’s the measure of how much sugar is in fruit juice or must. If you know how much sugar you start with, and how much sugar is left after fermentation, you know how much has been converted to alcohol, you can determine the alcohol content of your wine.
As fruit ripens, the BRIX increases and acidity decreases.