A blindfold covers a submissive's eyes. Typically they are made of soft leather and are padded by soft material on the inside which presses against the eyelids and prevents the submissive from opening their eyes. The blindfold is held in place by an elastic strap.
A blindfold causes a submissive or bottom to focus on what they're feeling and not be distracted by seeing what is going on. They also help a dominant or top to maintain an aura of mystery and surprise.