I had no idea what flossing was either before googling it (translates to 'tandtråd' in swedish). Basically it's a thin thread or similar you use to remove stuff from between your teeth, not easily accomplished while brushing. I believe moutwash will help as well.

Incidentally I also went to the dentist lately and I rarely brush my teeth more than once per day and I never floss but they thought my teeth looked good. My girlfriend flosses and brushes her teeth like a madman but she did not get any compliments for her teeth. I think it simpy varies from person to person and it's a lot about what you eat and drink.