This amazing poem is actually an elegy, which is a poem that mourns the death of someone or laments something lost. Dylan Thomas wrote this poem during the time of his father's death, and we can see that this poem above all urges his father to not yield submissively to its force, but to fight against the encroaching power of death. The poem above all states that those who are truly wise engage in this struggle even when they know that their defeat at the hands of death is inevitable, and as the poem ends, the poet expectantly looks for some kind of response to show that his father has heard his words and is struggling.
Structurally, what is fascinating about this poem is that it is a villanelle, which has a complex form involving the repetition of lines and also a fixed rhyme scheme involving only two rhymes in its nineteen lines divided into five tercets and one quatrain. The villenelle therefore is not an easy form to use, and yet Thomas manages to make the key repetition of the central lines of "Do not go gentle into that good night" and "Rage, rage against the dying of the night" appear to be spontaneous and not forced. In particular, the repetition of these lines adds to the cumulative intensity of the poem, making for a stunning and powerful final quatrain as these two lines are united:
And you, my father, there on the sad height,
Curse, bless, me now with your fierce tears, I pray.
Do not go gentle into that good night.
Rage, rage against the dying of the light.
The emotional intensity of this final stanza as we imagine the son next to his dying father, willing him to show any signs of resistance in the struggle against death is extremely powerful and poignantly moving. The central message of how we should struggle against the power of death is reinforced through this repetition.