Horses have teeth that erupt continually, and are ground down by the act of chewing their food. A horse's lower jaw is narrower than the upper jaw, so the teeth do not wear perfectly evenly. The outside of the upper teeth (next to the cheek) and the inside of the lower teeth (next to the tongue) often are not worn down like the rest of the tooth surfaces, and this results in sharp "points" which can pinch the cheeks or tongue and cause pain. Grinding these points down once a year will make your horse's mouth more comfortable so that he can chew more easily.
Other things that can go wrong in a horse's mouth include loose or fractured teeth, abnormal uneven wear such as "ramps" or "waves", or gaps between the teeth that pack feed material. All of these can be addressed during a routine dentistry appointment. Tooth problems in horses can have very subtle signs - even if your horse isn't dropping feed, or fighting the bit, the mouth should be examined yearly to be sure small problems don't turn into large ones.