Maceration is the winemaking process where the phenolic materials of the grape tannins, colouring agents (anthocyanins) aroma and flavour compounds are leached from the grape skins, seeds and stems. Maceration is the process by which red wine achieves its red colour, since 99% of all grape juice is a clear-greyish colour. In white wine making, maceration is either avoided or only allowed in a very limited manner. This is can be seen in the production of varietals with less natural flavour and structure like Sauvignon Blanc.
For Rose production, red wines grapes are allowed some maceration between the skins and 'must', but not to the extent of red wine production. The process of maceration begins as soon as the grape skins are broken and exposed to a degree of heat.

 

Temperature is the influencing guide, with higher temperatures encouraging more breakdown and extraction of phenols from the skins and other grape materials.
Cold maceration is a practice of cold soaking the skins of red grapes in their juice for a period of time prior to the start of fermentation. Temperatures of the must are kept low to encourage extraction by water and added sulphur dioxide rather than relying on heat and alcohol to act as a solvent.
Maceration continues during the fermentation period, and can last well past the point when the yeast has converted all the sugars to alcohol. During fermentation, higher alcohol levels can encourage this process with the alcohol acting as a solvent to assist in the breakdown of the compounds within the grape materials. This process seems to slow once the wine reaches an alcohol level of 10%.
Depending on the varietal, the process of maceration can enhance the body and mouthfeel for many wines. Greater extraction can add to the complexity and life expectancy of the wine by developing more complex tannins that will soften over a longer period of time. Care must be taken as too much extraction can also increase the harshness of some tannins to where the wine is not very approachable.