Starters are a more sophisticated form of using yeast.
Water with a small amount of flour and sugar in it is either inoculated with yeast, or allowed to become populated with yeast from the air. The result is a piece of dough called the “starter.” It is often quite pasty and moist.
This starter is used as the basis for bread, in which it becomes the leavener.
Before packaged yeast became commercially available for bakeries and household use, yeast starters were the only way to make raised breads.