These people are taking the huge statue which you see at the end of the truck to ceremonially dip into a river. This marks the end of a prayer or Pooja ceremony. In this case it is most probably a statue of Vishwakarkma. Otherwise it can be of Durga or Ganesh depending on the day of the year.

Notice the big black speakers at the rear where the men are dancing. They have also spread some colored powder in joy at each other.