A labret is one form of body piercing. Taken literally, it is any type of adornment that is attached to the facial lip. However, the term usually refers to a piercing that is below the bottom lip, above the chin. It is sometimes referred to as a "tongue pillar".
The root word is Latin labrum which means lip. The -et suffix means "something worn on". So combination of those two creates Labret - Something worn on the lip.