Sur lie means "on the lees" in French, and is a traditional white wine brewing method in the Loire region.
In typical white wines, the "lees" (dead yeast cells) that accumulate at the bottom of the tank after alcoholic fermentation is complete are removed.
However, in the lees fermentation process, the wine is left in contact with the lees, allowing the amino acids and other components in the lees to dissolve into the wine, adding complexity and umami to the flavor.
It is also said that the presence of sediment helps prevent the oxidation of wine.
Typical varieties that are used for sur lie include Muscadet and Koshu, but it is also sometimes done with Sauvignon Blanc.