Brix is the amount of the total soluble solids present in a batch.
Example: Sugar is a soluble solid because you can hydrate it with water then you could boil the water off making the sugar re-crystallize. Most soluble solids are from sugar and less from acid. The reason is because all sugars including sugar in the fruit is soluble.
Brix can be read quite easily with a refractometer. If you plan on doing a lot of new product development, I would highly recommend the purchase of a refractometer and pH meter. In summary, A brix reading is the amount of all soluble solids (sugar, acid, ect.) in a given mixture.