Before bottling, wines go through a process called clarification. As the name implies, it is intended to make the wine crystal clear and bright. Various things, including yeast cells, malolactic bacteria, bits of grape pulp and proteins can cause the wine to be hazy and these need to be removed. The old fashioned way of doing this was to allow the wine enough time for the solids to settle to the bottom and then to rack the clear wine off the top. This is still how most solids are removed. For roughly a century, winemakers have been using filtration to help with clarification. This involves passing the wine through a pad. Coarse ones have large fibres and only trap the largest particles, fine pads have much smaller, more tightly packed fibres and trap event the tiniest bacterial cell. A wine may need to pass through two or three pad filtrations of increasing tightness to get them to the required clarity. Some wines may only need a course filtration, while others may need a fine filtration for stability reasons. Every filtration is expensive, and invariably pulls out some flavour and colour, so winemakers try to filter as little as possible. For some, refusing to filter is a badge of honour that marks their adherence to traditional methods. To help you understand I'll go through the background of a trio of wines and how I would handle them.
1. Traditional barrel aged Shiraz - Course Filtration. The wine has spent two years in barrel and has been racked off solids a couple of times over that period. It is already crystal clear and is sound. It has no malic acid and no sugar, so no food for bugs. Some solids may be stirred up the last time it is pumped out of barrel, so it only requires a course filtration (or ideally none at all) to leave it clear and bright.
2. Modern off-dry Chenin Blanc - Sterile filtration. This wine is made quickly to preserve fruit freshness. This means that there is not time for the wine to fall clear of its own accord. Also, for stylistic reasons it has residual sugar and has not gone through malolactic fermentation. Hence, it has food for yeast and bacteria respectively. It needs to have every single yeast and bacteria cell removed by sterile filtration or the wine may start to ferment in the bottle - with explosive results! Now for the curly one...
3. Big and gusty Durif - Sterile filtration. A very big, super-ripe style, that has spent a couple of years in new oak. Although the wine has been made in the same way as the Shiraz, there is a fine distinction. Being super-ripe there may well be a tiny bit of sugar left in the wine that the yeast were unable to finish off. Also, there will be some sugars leached out of the wood of the new barrels. This tiny bit of sugar can be food for a spoilage yeast called Brettanomyces, which leaves the wine smelling horsey or of Band-Aid. Even if the wine is fine at the time of bottling, Brett can get to work in the bottle if it is not removed by sterile filtration.
Good winemaking is about interfering as little as possible, but this does not mean doing nothing. Zero, or coarse filtration will leave a little bit more character in the wine (and every bit counts), but failure to filter when filtration is required could leave you with a microbial time bomb.