Coffins are used in the Gravediggerrandom event. It is reported that the Gravedigger event takes place most often when players are burying bones. These coffins have been misplaced, and Leo asks the player to re-arrange the coffin into their respective graves.
To do this, players have to:
Remove the coffins from the graves.
Check the coffins to see their contents.
Check the grave markers to see the respective "owners" of the graves.