Jackson has a sole bruise, sometimes known as a stone bruise. A stone bruise can be caused by traveling over hard rocky ground, especially if your horse is not used to those kind of conditions. (It's just like when the weather gets warmer and you go out in barefeet for the first time!) A hard knock against a fence rail or a stall door, shoes that are too small or an overgrown shoe can also cause a sole bruise. 

A fresh stone bruise will cause minor lameness, for a few minutes or even a few days and are rarely serious. An old stone bruise may be seen when a farrier pares the horse's hoof - on a white hoof it will appear as a reddish-purple spot; on a dark hoof it may look like a dark gray spot. By the time you can see the bruise, the injury is already several months old.

How can you tell a fresh stone bruise from a more serious problem, like an abcess? Well, remember that a sole bruise usually only causes minor lameness which only lasts for a few days. There should be no heat or swelling of the sole, which usually is present with an abcess. Of course if you have any doubts, call your vet. 
Stone bruise