After the second fermentation occurs in the bottle the yeast dies and the deposits of yeast cells falls to the bottom. Non vintage champagne must spend at least 12 months on the lees and vintage champagne at least 3 years on the lees.
The storage on the lees play an integral part in the development of aromas, we find in champagne. The autolytic aromas which we percieve in champagne resemble brioche, nuts, bread or as biscuits.
The lees also adds texture, richness and finess to the champagne.