Tōrō (paper lanterns) are a traditional Japanese form of illumination, in which the flame is enclosed in a wooden frame wrapped with paper so that the flame it protects will not be put out by the wind. Tōrō nagashi is a custom by which fires are lit inside of tōrō, which are then floated down a river or the ocean. Taking place during the Bon festival, it is said to be based upon the belief that the spirits of the dead (the souls of one's ancestors,) ride down the river on the tōrō to return to the afterlife on the other side of the sea. Because of this, the offerings of light that are lit on the tōrō are said to be the light of wisdom, which illuminates darkness and uncertainty.
In Japan, during the Bon festival in which the spirits of the ancestors are said to return home, fires called mukaebi are lit to welcome the ancestors back and point them to the location of their descendants'home. In the same way, fires called okuribi are lit to send the ancestors on their way, so that their souls will not get lost on the road back. Tōrō nagashi is a part of okuribi, conducted so that the ancestors may safely find their way back to the afterlife, riding on pure water, sent off with wishes that they may be reborn into beautiful souls.
However, because of the differing regional interpretations regarding the period of Bon and the various opinions as to its origins, tōrō nagashi is not something that occurs in unison throughout Japan. Though there are some regions in which tōrō nagashi is not performed, most of the customs surrounding the period of Bon and tōrō nagashi are performed with a view towards respect towards one's ancestors and the peaceful repose of their souls.