Lees are the sediment at the bottom of a wine vat made up of things like dead yeast cells, grape seeds, pulp, skin fragments and tartrates.
Over time the yeast cells break down and this process termed autolysis releases nutrients into the wine which affect the smell, taste and mouthfeel.
If wine is left on lees for a short period it will have a very subtle affect of adding to the roundness and creaminess of the wine.
When it is left for long periods it will produce yeasty flavours of bread and vegemite as in bottle fermented sparkling wines that have aged on lees for several years. Lees aren’t always a winemaker’s friend as it can produce off flavours of hydrogen sulfide (rotten eggs) and it can make a warm climate wine too heavy and overbearing.
But some judicious lees aging on crisp cool climate white varieties can make them very enticing.