Simply: wines that have had suspended particulates resulting from the fermentation process removed. Important for future clarity and stability of a wine.
A process of 'cleaning up' a wine - used after fermentation and before bottling; it is similar to running coffee through a filter, but arguably not always necessary to produce fine wine. The purpose of filtering is to remove sediment, grape skins, dead yeast, crystals and bacteria that can adversely affect wine quality. The degree of filtering can range from very fine to coarse; however, it is increasingly being minimized (or avoided whenever possible) because the finer the filtering, the more flavours and characters (palate texture) are removed from the wine. Today - many wineries are using the more labour-intensive, old-fashioned practices of fining or racking to clarify wines. Historically, many filters before the 1980's were made from asbestos.
Filtration is also used to ensure clarity, plus physical stabilization prevents the formation of hazes and deposits after bottling, while microbiological stabilization eliminates yeasts and bacteria that can destroy a wine's taste. Careful use of precise filtering pads and agents allow the winemaker to target specific foreign substances based on their size.
Some winemakers feel that filtering reduces the quality of wine. Emile Peynaud, the preeminent University of Bordeaux enologist had this to say about the filtering debate:
Resistance to the practice of filtering arose from the reproach made that it tended to thin down and emaciate the wines. Nevertheless, if every precaution is taken - it may be stated that the mechanical action of filtering has never had a negative influence on quality. To suggest the contrary would mean conceding that the foreign substances - which filtration is precisely designed to remove, have a favourable influence on taste.
Filtration is also a key stage in the elimination of the deposits formed in sparkling wine during its second fermentation in the bottle.