Enzymes from the yeast cells attack starch, breaking it down into glucose. Other enzymes transform glucose molecules into carbon dioxide and ethanol. Breads which are leavened by baking powder instead of yeast lack the tasty molecules of fermented bread. That's because when baking powder gets wet, a chemical reaction occurs that releases only carbon dioxide, salt, and water. In breads leavened with yeast, however, the yeast cells grow under anaerobic conditions and cannot convert glucose molecules completely to gas.
Salt strengthens gluten by slowing down the enzymes which catalyze the breakdown of proteins.
If you add too little salt, the dough is tough and sticky. If you add too much, water flows out of yeast cells by osmosis. Then nutrients are lost and production of carbon dioxide slows down. After dough rises a couple of hours in a warm place, it's ready to go into the oven. There, heat causes pockets of gas in the dough to expand.