Lanterns

The Lanterns of the film Tangled, appear at the festival for the lost princess, in hope that they will guide her home

Tangled
The lanterns start out as a celebration for the baby princess Rapunzel. The King and Queen launched one into the sky, and it rose up into the heavens. When Mother Gothel stole the baby to keep herself young and beautiful, the lantern launching tradition began. Ever since then, on the princess' birthday, they launch thousands of glowing lanterns into the sky in hope that she will be guided home by them. While growing up, Rapunzel was fascinated by them and wanted to see them in person. She does end up seeing them with Flynn Rider and is fascinated by them.

Tangled: The Series
The lanterns play another role in the series. In "The Quest for Varian", a lantern attaching a letter appeared in Rapunzel's nightmare. Later, Rapunzel found the same lantern alongside her statue. The lantern left a message and a key from Varian to seek Rapunzel's help.

Once Upon a Time
In the show, they are lit by Anastasia and Drizella, who are in hopes that they will be reunited with their mother Rapunzel, after she is trapped within a tower by a witch, and believed to be dead.

Trivia

 * The lantern that King Frederic and Queen Arianna send into the sky has Corona's sun crest. They use that style of lantern to start the festival off, before the rest of the kingdom send up their own lanterns.
 * After receiving a lantern from Varian in "The Quest for Varian", Rapunzel referred to it as a kite.