Any atkins diet help?

my friend is doing the atkins diet, and so i wanna do it. She said that I'm only aloud to have 20 grams a day. ummm.. 20 grams of what? I'm not sure so i looked on the back of the backet of cheese and it said '21 grams" then it said "7grams" does it mean 20 grams of net carbs? and if so, what are net carbs and how do you identify them on food label info. Thank you for all your answers.