For our uses, percent soluble solids is a measure of the sugar content of the grape juice. The higher the sugar content the sweeter the juice, so in general we want a higher percent soluble solids.
Soluble solids percentage will differ based on cultivar, berry ripeness, crop load, and environment. We measure percent soluble solids by taking a sample of 10 berries and squeezing the juice onto an electronic refractometer. Our readings are the average of several years, and we try to measure at the time the berries would be just ready to pick. I have found that you want a soluble solids percentage of at least 14 or 15 to have a good flavor, and berries at 17%+ are noticeably very sweet.