Sediments are more often found in red than in white wines. An unfiltered red wine will have been stored in a barrel for a year or even more, and during that time fragments of grape skin, yeasts and crystallized tartrates will settle to the bottom of the barrel. The winemaker then carefully racks off the clear wine from above the sediment, and the wine is bottled. Although initially quite clear, a sediment will start to form in the bottom of the bottle. Some of this is fine fragments of skin or yeast that remained in suspension during bottling, and some comes from polymerization of soluble tannins which have been extracted from grapeskins and oak barrel walls. Winemakers accept the deposition of sediments in wines designed for long aging in bottle: they help develop the aromatic compounds that constitute bouquet, and polymerization of the tannins removes the drying mouthfeel given by tannins in a young wine, and helps the wine develop softness on the palate.
For these reasons, wine sediment is often seen as a sign of quality. It helps to give the wine character and complexity, and is not a fault. Sometimes the sediment will remain firmly in the bottom of the bottle, or you may wish to separate it by carefully decanting the wine into a decanter, leaving the last few millilitres in the bottom of the bottle.