What is the meaning of soup sud?

"Soup sud" typically refers to the foam or bubbles that form on the surface of soup or broth during cooking. This frothy layer is often composed of fats, proteins, and impurities that rise to the top as the liquid heats. In culinary practices, it is common to skim off the soup sud to achieve a clearer, more refined broth. The term may also be used informally to describe any frothy substance in liquids.