Lees is the heavy sediment in a wine fermenting vat or barrel and consisting of dead yeast cells and other solid matter...grape skins, pulp and seeds. Exposure to the lees imparts flavor and body to a wine. As a result, aging a wine on the lees (sur lie), and stirring it occasionally increases flavor and body. Wine is separated from the lees by the process of ‘racking’.  Any remaining solids may be removed by the process of filtration.

References to lees go as far back as the early Jewish scriptures where the phrase "Wine on the Lees" is used in Isaiah 25:6 in the King James version of the Bible.