YAN stands for Yeast Assimilable Nitrogen. Nitrogen is probably the most important macronutrient for yeast after sugar, and is needed to carry out a successful fermentation that doesn't end prior to the intended point of dryness or sees the development of off-odors and other wine faults.

YAN is the combination of Free Amino Nitrogen (FAN), ammonia (NH3) and ammonium (NH4+) that is available for the wine yeast to use during fermentation.

The minimum requirement of YAN for a healthy fermentation should exceed approximately 150 mg/L.

Total YAN should not exceed 400 mg/L since yeast might fail to consume more than this amount, which can create a hazard as other spoilage organisms besides beneficial wine yeast can utilize the nutrients. 

The YAN content should be determined by chemical analysis before making any nutrients addition to the grape must or juice.