Here's what your horse's ears tell you about what he is thinking.
For all the verbal chatter you may aim at your horse, the meaningful dialog between you and him is purely physical. From the most basic handling requests to the most complex command, your only understandable messages are conveyed by your body through direct contact, posture or gestures.
Naturally, a horse will speak his mind during this physical dialog. In fact, by keeping an eye on your horse's ears the next time you ride, you'll gain new insights on just how well your mount is "hearing" what you have to say from your place in the saddle.
The "listening" horse carries his ears in neutral, neither far forward nor flat back. They are relaxed so that they bounce with his movements and are cocked slightly to the rear toward his rider. During circles and turns, the ear to the inside of the bend usually will swivel rearward as though to tune in to the aids being applied primarily to that side. Though there's nothing to hear, the horse naturally directs his ears, separately or together, toward the focus of his attention.
The straining, forward-pricked and flopping half-mast ear positions indicate horses who have gone temporarily deaf to their riders' requests, the former having discovered some outward interest that doesn't include you and the latter having fallen asleep. And what about the horse who pins his ears while being ridden? He has heard perfectly well what you (or some nearby horse) has said, and he takes strong exception.
Condensed from Understanding Equine Behavior