Bumblefoot is a bacterial infection of a cut or abrasion on the bottom of the foot. The bacteria may be staphylococcus, or one of several other possibilities. The infection might resolve on its own, or it might become a large, severe, painful lesion. The bacteria could eventually get into the bloodstream and cause severe illness and death. So the upshot is to treat it as soon as you notice it, even if mild. The milder it is, the more likely you will be able to treat successfully. Some cases might resolve spontaneously, but I would not risk it.
I've had 3 cases and successfully treated all of them non-invasively, as soon as I noticed them, while still mild-moderate. For mild-moderate cases, soaking is often helpful. Some report success with epsom salts. I soaked with an antibiotic known to be effective against staphylococcus, as I describe here:
http://ouroneacrefarm.com/bumblefoot-treatment-tricideneo/
Surgery may be the only treatment option in more severe cases. Basically, if it's so bad that a hardened kernel has formed within the swelling, the kernel needs to be removed surgically. Soaking won't make that go away. But don't take surgery lightly. Some people report failure with surgery, requiring multiple attempts and a lot of pain for the bird.