Literally translated, «cuvée» means something like «out of the tank» and, in the case of champagne, refers to a specific batch bottled separately or a specific blend from a harvest or vintage.
One meaning of this term is thus the measure of quality of the wine obtained from the very first, very gentle pressing of the grapes. The best champagnes are made entirely from cuvée. But cuvée champagne can also refer to the specific blend of wines (precisely from different «batches» of a harvest) that go into a champagne house's special recipe.
Only the cuvée is considered to be of high enough quality to be used for the production of champagne. If the harvest yields further must (the taille), this may only be distilled.