The term 'dosage' refers to the amount of sugar added to champagne just before the final closing of the bottle. In case of 'brut' a maximum of 12 grams of sugar per liter is added, 'extra brut' will contain no more than 6 grams.
Champagne used to be a sweet drink, but these days champagne lovers prefer it dry. More and more champagnes have no dosage at all: 'brut nature', or 'zero dosage'.