What they are
“Net carbs” are the carbohydrates left after subtracting fiber and sugar alcohols from your total carbohydrate intake. In other words, “net carbs” are starch and sugars. Some people prefer to track net carb intake instead of total carbohydrate intake because they’re tracking carbohydrate intake for the purpose of roughly estimating the glycemic response impact of their diet (fiber and sugar alcohols have minimal direct glycemic impact). Oftentimes, they’re pursuing a ketogenic diet – fiber and sugar alcohols won’t take you out of ketosis, but starch and sugar (above a certain intake threshold) can.
For more on net carbs, see the entries on starch and sugars.