Light and dark are used metaphorically in the poem to represent wisdom, enlightenment, and guidance, as well as ignorance and death and the struggle to stay alive against the inevitable onslaught of death. The speaker implores the old man: "rage, rage against the dying of the light" (line 2). This repeated line emphasizes the struggle against darkness and the speaker's plea for the old man to resist death and fight for his life. The "light" in this line is not just the physical light of day, but a metaphor for life and vitality, the strength to face death with dignity.
The poem's imagery also emphasizes the contrast between light and dark, and the ways in which they can coexist. The speaker refers to the "fierce" and "wrathful" light of the dying sun, which is "burning in the heart of the stars," suggesting that the light of life can be both bright and intense, and yet also fierce and destructive. This imagery suggests that the old man should embrace both his fight against death and his acceptance of the inevitability of death.
In the final stanza, the speaker says that "wise men at their end know dark is right," and yet they can "bless the candle of the body" and "honor," suggesting that the light of life is precious, even in the face of death. This suggests that the speaker embraces both the light and dark, recognizing the importance of both life and death. The poem encourages readers to embrace life and fight against death, but also celebrates the beauty and dignity in accepting the inevitability of the dying of the light.