A blinder is a flap attached to a horse's bridle that partially blocks its side vision. Blinders (also called blinkers) help horses stay calm and focused by preventing them from seeing distracting or frightening things beside or behind them. A horse pulling a carriage through busy city streets might wear blinders so it won't spook at passing cars or sudden movements in its peripheral vision.

Carriage horses, plow horses, and racehorses often wear blinders to keep them moving steadily forward without getting distracted or scared.

When people say someone has blinders on, they mean that person is ignoring everything except one narrow view of a situation. A student with blinders on about their favorite sports team might refuse to admit when the team plays poorly. Someone planning a camping trip with blinders on might pack perfectly for sunny weather while ignoring the forecast that shows rain all weekend. Having blinders on means you're so focused on one thing that you're missing important information all around you.