Wind puffs generally fall under the heading of 'blemishes'. Usually the sign of a horse that was worked too heavily for too long or was trailered a lot. Usually not anything but a cosmetic issue in most cases. Occasionally some horses just have them for no reason whatsoever. If there is no lameness problems, I'd largely ignore them unless they get really big.
Nominally speaking, it's just the bursa in the joint expanding due to excess synovial fluid in the joint bursa. Or it could be the way the horse is put together. I see a lot of wind puffs on former race horses and hunters/jumpers without there being an issue other than cosmetic.
But, on the other hand, it could he a good indicator of a horse that was worked beyond its capacity to recover adequately from that stress.
Here's a good page that has some information buried in it: http://horses-arizona.com/pages/arti...dfeetlegs.html