Lingual braces are invisible because they are placed on the inside (tongue side or lingual side) of the teeth. But invisibility is their only benefit over traditional or clear braces. They tend not to be as comfortable, more difficulty to keep clean and come with more dietary restrictions.
Treatment with lingual braces does tend to take a little longer, appointments are longer and orthodontic fees are higher. But for patients that want orthodontic treatment with truly invisible appliances, this is often the best choice.