This poem was written to Thomas' father whose eyesight and health were failing at the time. Thomas may also have been thinking of himself, for as a teenager he had an illness and his doctor gave him four more years to live. Indeed, this poem was published in 1951, and Thomas died in 1953 at the age of 39, due in part to alcoholism and pneumonia (it's pretty complicated. Look him up in Wikipedia if you're curious).
In this poem Thomas does not deny that death is inevitable, but he presses the belief that all men should fight it as best they can. Although I agree that people shouldn't just give up on life, Thomas' father was in his eighties and it seems like he had a pretty good run. For me, a person who believes in God and heaven, it seems a little silly to try to prolong something when the next step is going to be much better. However, I certainly understand Thomas not wanting to see his father become frail and eventually leave him. I'm not looking forward to that with my parents either (and I plan on outliving them).
The Poem
Do not go gentle into that good night,
Old age should burn and rave at close of day;
Rage, rage against the dying of the light.
Though wise men at their end know dark is right,
Because their words had forked no lightning they
Do not go gentle into that good night.
Good men, the last wave by, crying how bright
Their frail deeds might have danced in a green bay,
Rage, rage against the dying of the light.
Wild men who caught and sang the sun in flight,
And learn, too late, they grieved it on its way,
Do not go gentle into that good night.
Grave men, near death, who see with blinding sight
Blind eyes could blaze like meteors and be gay,
Rage, rage against the dying of the light.
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.