put simply, your fish poo. that poo breaks down into ammonia which is toxic to fish. imagine breathing your own poo.
one type of bacteria eat the ammonia. those bacteria then poo and create nitrite. nitrite is also very toxic to fish.
a second type of bacteria (nitrobacter) start growing. these guys grow more slowly but they eat nitrite. when nitrobacter poo, they produce nitrate. nitrate is less toxic & perfectly safe for fish until it gets higher than 20-40 ppm.
there's no reason to add BB once you're cycled unless you add new critters & wanna help your BB grow to catch up.