Little peices of metal that goes on each tooth that being worked on. Along with a wire going through each peice of metal. the braces they a device which are in orthodontics, which are used to apply force on the tooth for movement to the desired direction..they apply force. Definition of Orthodontic treatment (dental braces)
Orthodontic treatment (dental braces): The use of devices to move teeth or adjust underlying bone. The ideal age for starting orthodontic treatment is between ages 3 to 12 years. Temporomandibular joint (TMJ) problems can be corrected with splinting or dental braces. Teeth can be moved by removable appliances or by fixed braces. Crowding of teeth can require extraction of teeth. Retainers may be necessary long after dental braces are placed, especially with orthodontic treatment of adults.
and it's what ARE braces? they are referred to in the plural.

Teeth - orthodontic treatments

Orthodontic treatment is often recommended to correct abnormalities in jaw and tooth position, such as an overbite or protruding teeth. An overbite is when the top teeth bite over the bottom teeth more than normal. In extreme cases, the edges of the front teeth can damage and strip away the gum and loosen the lower teeth or the edges of the lower teeth can cut into the roof of the mouth.

Protruding, or ‘buck’ teeth is when the front teeth are positioned further forward than normal. A person with protruding teeth is more susceptible to tooth loss from accidents and may suffer from speech or eating problems. Many people also choose to correct protruding teeth for cosmetic reasons.

These problems should ideally be corrected in adolescence. Ask your dentist to recommend an orthodontist for advice on these conditions.