That's commonly known as bumblefoot. Staph bacteria gets in a wound on the foot from a puncture from things like sharp rocks or a splinter/thorn. Inject 1/2 cc of Penicillin or Duramycin directly in the swollen area. Use a 22 gauge needle as I can't get Penicillin up into any gauge smaller. It looks pretty advanced so you may have to cut the abscess and squeeze the fluid out. Make the incision on the upper side of the swelling. Fill the opening with iodine or Staphaseptic (better than Neosporin), and wrap the foot with a clean cloth. Use some surgical tape/sports tape to keep it in place. Don't wrap it too tight or else you'll cut off the bird's circulation. Keep the bird in small coop with soft flooring and no roost for at least a week and redressing the foot a couple times during that week until it is healed. Bumblefoot is a tough one to deal with, but it can be done.