Sur lie is French for "on the lees," the lees being the dead yeast cells and other grape particles that are a by-product of fermentation. A winemaker will sometimes allow a white wine to age with the lees for several months before bottling it directly from the fermentation tank or barrel, bypassing the racking or filtering process. It's done to add complexity and flavor to the wine. Chardonnay is often made using the sur lie technique.