The phrase "rich source of dietary fiber" means that a particular food contains a lot of fiber, which is a substance found in plant-based foods that helps with digestion and keeps you healthy. Full definition
White Beans are a super rich source of dietary fiber, which is good for the prevention of constipation, also helps digestive disorders like irritable bowel syndrome and diverticulosis. (cali-zona.com)
Chia seeds are a very rich source of dietary fiber, two tablespoons contain about 10 grams of fiber. (ahealthblog.com)