If you want your fermentation to be as optimal as possible, you might consider using yeast nutrients. But what exactly is it and in what situation is it recommended?
Yeast gets its nutrition largely from the sugars we manage to extract from the malt during mashing. But these are mostly sugars, and yeast cannot live on that alone. Fortunately, there are also all kinds of other things in the wort that yeast cells ultimately benefit from. These substances such as minerals and vitamins are mainly present in the wort and come from the malt.
Nevertheless, it can sometimes happen that the nutrient medium for the yeast is not optimal enough. As a result, fermentation can stall and really, in these situations, you want to have the option of adding some additional yeast nutrient. This will keep the yeast performing well and prevent fermentation from stalling in a fairly inexpensive way.