This Yeast Nutrient is a blend of vital minerals/nutrients that aid in yeast activity during fermentation. (Zinc, Phosphorous, Potassium and Manganese)
The recommended dosage is 4-10g per 100L. It should be slurried in a few liters of liquid and added to the kettle about 15 minutes before the end of the boil.
This is highly recommended when making yeast starters with DME. DME does not contain vital nutrients for yeast health, like zinc. It is beneficial to add yeast nutrient to your yeast starters.

Why use Yeast Nutrient?
- Aids in boosting yeast activity during fermentation
- Can reduce off flavours in beer that are caused by bacteriological contamination.
- Can help push your final gravity lower by having more active yeast near the final part of fermentation.
All of which can reduce diacetyl content in your finished beer.

The benefits of using yeast nutrient are described in the pdf and video below:

Lallemand - Feeding Yeast for Brewing Success!