That a person is dead means s/he is socially dead, “the final separation of a person from a human community” (Carey, Communication as Culture).
For living people, all kinds of rituals serve to prolong the death, which can reduce the sharpness of the break between life and death, preserving the continuity of community existence.
But for the dead people, how can they know they are dead already? That is what the movie tries to answer, although not very successfully.