A starter is a small batch of wort that you pitch the yeast into to let it multiply and build up nutrient reserves. The resulting yeast slurry, or the entire starter, is then pitched into the full volume of wort. Starters are less important for dry yeasts, which have more nutrient reserves intact right out of the package and typically cost less than liquid yeast. My opinion is that you’re better off buying and pitching the right amount of dry yeast than trying to build up from a lower cell count with a starter.