Dylan Thomas's poem conveys the concept that men, especially those nearing death, should not accept death passively but fight back against it as hard as they can in order to stay alive as long as possible. In urging his father in the strongest possible terms to resist dying, the speaker reveals his grief at the possibility his father will soon pass away.
Thomas compares old men to bright lights, using images such as lightning that "forks" and meteors that "blaze" to describe their life force. He urges his father to use this vibrant life source to "rage" against death and the darkness it brings.
The poem pushes aback against stereotypes that characterize the elderly as passive, frail, and spent, simply awaiting the end. He depicts them instead as filled with anger and passion, as those who "rave" and "curse."
The speaker says that "wise" older men can know their death is coming but still fight against it with all their strength. They don't have to be resigned to the inevitable but can struggle to hang on to what they have.
By forcefully advising his father to fight back against death, the speaker shows how much he values his father's life. He shows that he wants his father, by engaging in life fully, to stay with him as long as he possibly can.