The second of the two repeated lines in Dylan Thomas's poem "Do Not Go Gentle into That Good Night" reads
Rage, rage against the dying of the light.
The "dying of the light" is death, as is "that good night" with which it is constantly paired. No one can say exactly what the final moment of death is like, but, partly by analogy with sleep, it seems obvious that you will stop seeing the things around you. Your vision will disappear as the light goes out of your eyes. The "dying of the light" is, therefore, an apposite description of the physical process of death.
It is common for people who are dying to struggle for breath. This struggle may prolong life for a few moments, but whether it does or not, it is something one could actually do. If your eyes grow dark, then all the "raging" in the world will not make any difference. The poet, however, does not want his father to resign himself to the inevitability of death. He wants his mental attitude to be defiant and fierce, clinging on to life for as long as possible. This refusal to accept the "dying of the light" is important because it mirrors the poet's own attitude, and his urgent exhortation allows him to project this defiance into the mind of the dying man.