Once fermentation finishes, the yeast that did the work dies and settles as a sediment. Whether a winemaker removes that sediment or leaves the wine sitting in it shapes the texture of the finished bottle.
What the lees actually are
Gross lees consist of dead yeast cells mixed with grape solids and pulp fragments. They are heavy and settle quickly, usually within days of fermentation ending.
Fine lees are the lighter fraction of yeast cells that stay suspended much longer. These are the ones used deliberately for ageing, and they are what a winemaker means by the term.
Most producers rack away the gross lees promptly, since that heavy layer can develop reductive and unpleasant aromas. The fine lees are then kept for months or longer.
Autolysis releases compounds into the wine
Dead yeast cells break down over time in a process called autolysis, releasing mannoproteins, amino acids and other cell wall material.
Mannoproteins bind to tannins and other compounds and increase the perceived viscosity of the wine, which registers as a rounder, fuller mouthfeel.
The same compounds contribute a savoury, bread-like aromatic character. In sparkling wine aged for years on lees it becomes the dominant signature.
Stirring accelerates everything
Lees stirring resuspends the settled cells through the wine, increasing contact between the liquid and the breaking-down yeast. It also briefly introduces a small amount of oxygen.
Winemakers pursuing texture stir weekly through the ageing period. Those wanting a lighter effect stir occasionally or not at all and let time do the work more slowly.
Overdoing it produces a heavy, slightly flat wine where texture has overtaken freshness. The technique is easy to push too far.
Lees protect as well as build
Yeast cells consume oxygen, so a wine sitting on its lees is chemically shielded against oxidation without additional sulphur dioxide.
This lets producers work with lower sulphur additions during ageing, which is one reason the practice appears frequently in low-intervention cellars.
The protection cuts both ways, since heavy lees in an oxygen-starved vessel can generate sulphurous reduction faults instead.
Where it shows most clearly
Whites fermented and aged in barrel almost always sit on their lees, because barrels are difficult to rack frequently and the two practices grew up together.
Sparkling wines made by a second fermentation in bottle rest on lees for years, and that duration is the main variable separating styles within the category.
In still whites, the tell is a broad, slightly saline texture that persists after the fruit has faded from the palate. It is structure rather than flavour.