Bumble foot is when, something punctures the chickens foot, and it heals wrong and causes infection.......It usually happens on the bottom (foot pad) of the chicken's foot. I think it happens when the chicken steps on something, like a thorn.....Anyhow, you'll notice a chicken with this limping, and when you look at the bottom of the foot, you usually see a black scab.
I've actually had to do a surgery for this. Under the black scab is an actual white core....You literally have to cut that out. Chickens tolerate the surgery very well. I wrapped my hen in a towel, and covered her face. My husband held her firmly with her legs over the sink, her body was on the counter. I used a sterilized exacto knive and dug out the core. Flushed the wound with cold water and then put some scarlet oil on the wound, and wrapped the hen's foot with vet wrap.
Unfortunately, alot of the times, the bumblefoot comes back. If you don't try and get it early, it can really get bad......
I hope your chickens never get it, but if they do, there are great instructions here on this website with pictures.....