When round penning your horse you can step back and draw a horse into you. Posture carries over into the saddle. If you stay relaxed and don't over react you will teach your horse to be calm. When you know what to look for the posture of the horse can tell you whether you are making progress or how the horse feels. Since you can't ask them if they understood something or what they are thinking, this is a valuable tool.
Posture can be as subtle as the worried wrinkles around they eye or as obvious as a strike or kick.
Experience is the best way to learn to read the horse's posture. Posture is situational, which means that the posture you see in a certain situation may not apply to a different setting. There is some risk involved in horse training for both you and the horse. Horses can cause serious injury. Be sensible and don't attempt anything that is outside your comfort level.