To ferment. What does this mean to you?!
Answers: Mixing yeast and sugars (in the right environment) to create alcohol. The yeast flourishes feeding off the sugar, and the byproduct is a form of alcohol. It takes a certain amount of time for this to happen to completion (e.g.: not instantly).
To turn from grape juice to wine
Fermentation is when yeasts metabolize sugar in an anaerobic (no air) environment. This produces carbon dioxide gas and is the reason that bread rises, and that beer and champagne are "fizzy."