With most liquid yeasts, the use of a starter is necessary in order to create a viable amount of pitchable yeast. Even yeasts marketed as directly pitchable can benefit from the creation of a starter. A starter reduces "lag-time", before fermentation commences, and can help prevent a stuck fermentation. A typical starter involves fermenting a vial or package of liquid yeast in an small amount of wort a few days before brewing.
Dry yeasts do not require a starter. Making a starter with dry yeast can be detrimental to their performance because they can use their nutrient reserves before pitching.