Net carbs refers to the number of carbohydrates in a food that your body will assimilate.
Take the total number of carbs and subtract any carbohydrates from fiber.
Some people are also able to subtract the carbohydrates from sugar alcohols like Isomalt and Maltilol. Some people are sensitive to sugar alcohols and have to include those carbohydrates in their daily carbohydrate allowance.
So, as an example for you, sunflower seeds:
Total carbohydrates: 5 grams
Dietary fiber: 2 grams
Net carbohydrate content: 3 grams per serving