They're blinkers and they block the sideways view of the horse so that it can only see in front of itself. They help to reduce distraction. Horses in the wild are predated on and have eyes on the sides of their heads so that they can see almost 360degrees (for spotting predators), the blinkers reduce the field of vision so that they can only see in front of themselves. Predatory animals, like cats, dogs and humans, have eyes on the front of their heads which allows them to judge distances more easily.