If you can imagine a boil on the bottom of the foot, usually with a black scabby bit like a stone bruise, then you have bumblefoot. It is in fact a sort of "stone bruise" , in that it usually starts with a bruise to the sole of the foot when the hen lands on a hard floor, although not exclusively caused this way. The foot can become infected around the bruise, particularly if the bruising is repeated over time. Because they are deep seated, they are quite hard to treat, and of course hard to keep clean. They can also be caused by the perches being uncomfortable for the hen to roost on.
It is normal for the sole of the foot to be a little "puffy"