Natto is fermented soy beans. The beans are boiled, wrapped in straw and left to ferment for several days. They have vitamins, fiber and protein and thus are a good source of nutrition.
Natto dates back at least 1000 years and eventually became a favorite of people living in Edo, the capital city of Japan at that time.
People sold natto basically door-to-door. Some people added it to miso, others added minced onions and soy sauce and then poured the entire sum over some hot rice. It's cheap and has a rather strong odor.
Natto is sometimes used as a breakfast food, mixed with beaten raw egg and soy sauce, then poured over steaming hot rice. Natto is definitely not a favorite of non-Japanese, though, and even many Japanese do not care for it.