That is probably bumblefoot.  Some chickens get it, and it doesn't become a problem, but it is is red, swollen, or the chicken is limping, it needs to be taken care of, usually with the surgery to remove the pus core.  I have a lot of tiny sharp rocks in my soil around the coop, and many have those bumblefoot spots.  I do the surgery if it is bad looking, but sometimes they don't pose a problem.  You should look at the foot periodically to see if it needs to come out. Some people have used epsom salt soaks, or a product called Tricide Neo to treat it, and some give penicillin, but surgery if the quickest cure.  Here is a good link with a video of the surgery:  
http://www.the-chicken-chick.com/2011/07/bumblefoot-causes-treatment-warning.html