Horseshoes are things worn by horses to protect their hooves (feet) when they walk - like a shoe. Most horseshoes are made of iron or steel. Some are aluminum, rubber, plastic, rawhide or a combination of materials.
A farrier is someone who makes horseshoes and nails or glues them on the horse's hooves. Using nails does not hurt the horse, as the outer part of the hoof cannot sense pain.
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 one tries to throw them on a poll.