two pieces of leather that are put at the side of a horse's eyes so that it can only see forward

Blinders prevent a horse seeing behind it.

So why are blinders so common on the racetrack?

Blinders help the horse to concentrate on what's in front of them.

He puts his draught horses in blinders, so they follow his lead and don't get spooked by sudden movements.

They campaigned to have blinders removed from working horses.

See also: blinder (HORSE EQUIPMENT)