Dewclaws are the small, often higher-set "thumbs" found on the inside of a dog's leg, usually above the paw. They are not involved in walking and can sometimes be removed surgically, especially if they are prone to getting caught or injured.
However, some Border Collies may have dewclaws that are double dewclaws, meaning they have two on each leg instead of one. This is less common, but it is a trait that can be found in the breed.