Spurs are metal yoke with a cog worn on riding boots. They help the rider control or activate the horse.
Spurs are an accessory designed only for experienced riders. They come in different sizes, with ornaments, and some of the models might have a safe trim in shape of a rubber ball. You should buy spurs if your horse requires them, but use them very, very sensibly, as wrong use of this item might bring you way more harm than good.