If a horse has a lot of miles hes kinda been there done that. A horse with fewer miles may be young and inexperienced
That it's a horse with a lot of experience in it field of expertise and most likely did quite a bit of travel for the experience.
Means that it has been to a lot of places, seen a lot of things and has experience in a wide range of things.
An experienced horse in showing/trails/lessons etc, depending on what they say it has miles in