A Triforce necklace is a necklace made in the image of the Triforce symbol from "The Legend of Zelda" video games. The Triforce symbol stands for the Golden Goddesses who originally created the kingdom of Hyrule where the Zelda games take place.
The Triforce symbol is made up of three triangles, each of which represents its own Triforce. The three Triforces stand for wisdom, power and courage. Each of these Triforces can be split into smaller pieces, as evidenced in the video games. Sometimes the pieces of the Triforce manifest on a character's hand; when two characters who bear the Triforce symbol come together, their Triforce symbols glow.