Horseshoes are things worn by horses to protect their feet when they walk - like a shoe. They are made by a person called a farrier who makes them and nails or glues them on the horse's hoof (feet). Horseshoes are usually made from iron, rubber, plastic, rawhide or a combination of these.
People also put horseshoes over doorways, because a long time ago, many people thought it was a sign for good luck and it protected whoever walked under it from evil spirits. Horseshoes are also used in a game where you try to throw them on a poll.