Definition - What does Natural Yeasts mean?
Natural yeasts are those which are naturally found on grapes. These are also referred to as wild yeasts as they exist naturally in the environment where the grapes grow. Each wine region will have their own strain of yeast. Some wineries choose to use natural yeasts to start fermentation, however they are not always a reliable source for fermentation.
WineFrog explains Natural Yeasts
Natural yeasts exist everywhere. The white coating on grape skins called bloom, is a waxy substance which allows yeast in the air to attach itself to the grapes. Upon harvest, the grapes are processed and sent to fermentation vats. Following a cold stabilization process which usually only lasts 24 hours, the winemaker has a choice to allow the wine to warm up naturally and allow for the natural yeasts to begin fermentation or to inoculate the must with another strain of selected yeast.
Some believe that the use of natural yeasts makes for a more complex and "local" wine, while others feel it is unreliable. The selected yeast strain added to the must is a stronger strain than a wild yeast and will, thus, take over the fermentation. A winemaker is often always sure of the resulting wine with selected yeast rather than allowing natural yeasts to ferment the juice. It usually comes down to a matter of personal preference.