It means that the total carb count is 30.3g and out of that sugars total only 2.1G

But seriously that doesn't mean it's automatically OK, you need to see what the remaining 28.2g of carb are. Depending on the product, they could be wheat (a nono) or perhaps artificial sweetener such as maltitol, in which case you need to experiment to see if they effect your weightloss or not - it's a case of Your Mileage May Vary with a Vengeance

When in doubt, read the ingredients list on the packet very carefully. If wheat or sugar in any form come high (ie near the front) in the list, then it's probably best to avoid the item.

Hope this helps.