1) Organize all the files while on set. Basically, they will link the sound files with the video files together.
2) They will back-up all footage onto other hard drives to make sure nothing is lost.
3) They begin to do a light, unfinished color grade/correction so that the director and cameraman can get an idea of what the final film will look like. This helps to let the director and cameraman know if they should adjust their style of shooting while on set or not