Brewing the best beer possible means using enough yeast to get the job done. Unfortunately, a single vial or pack of liquid yeast contains only enough cells for very low gravity ales, up to about 1.030. Although you could simply use multiple packages, this can get expensive if you make lagers or high-gravity ales.
But yeasts are living organisms, and given nutrients and a food source, they’ll happily reproduce. Homebrewers can use this to their advantage by making a yeast starter. A properly made starter lets you build up the number of yeast cells you need from just one package and can save you money.
A starter is simply a small volume of wort that’s used for the sole purpose of growing yeast cells. It takes only about half an hour, but plan to make it at least 24 hours before you need the yeast. This will give the yeast cells time to reproduce.