This is a poem that talks about aging and eventually, going against dying. The good night is a metaphor for death and dying is symbolized by going into the white light. This poem is about the struggle of those who are becoming closer to death because of their age.
The author insists that people should go against the light, to rage against the light so that they can live longer. The overall theme of the poem speaks of grief. It insists that his father should stop going into the light and should shun the light away. This is the natural response of someone who has been left behind because of death.