Hard to Explain

Have any of you ever seen Top Gear? At the end of the year they came up with a Vietnam special where they went up the whole of the country in motor bikes.

At any rate, during one segment of the show they are at some restaurant or whatever, and on the river there were a number of flower looking things floating along, lit up with all kinds of different colors. I think it's a yearly event.

Does anyone know what this is and what it means?