What is this?

What is galling? My 21-month-old gelding has an infection just on his right hind leg, on the outside. It is yellowish, probably from puss. I asked a question about that spot, and was told it was galling. I looked it up online, and all I found was galling from the cinch rubbing along the horse's girth. My horse has not been broke to ride yet, and hasn't had a saddle on in quite some time (too cold outside!). So, i was wondering how my horse would get it on his leg? just a question. If you look at my other recent question, it has a link to the pic of his leg. thanks!