Tōrō nagashi (灯籠流し) is a Japanese ceremony in which participants float paper lanterns (chōchin) down a river; tōrō is traditionally another word for lantern, while nagashi means "cruise, flow". This is primarily done on the last evening of the Bon Festival festival based on the belief that this guides the spirits of the departed back to the other world.
The ceremony may be done on some other days of the year for other reasons such as to commemorate those lost in the bombing of Hiroshima and those who died on Japan Airlines Flight 123; or in other areas of the world, such as Hawaii, to commemorate the end of World War II. The Bon Festival takes place on the thirteenth to sixteenth of August or July, depending on the calendar you go by. The white lanterns are for those who have died in the past year. Traditional Japanese beliefs state that humans come from water, so the lanterns represent their bodies returning to water (traditionally the sea in this case).