"Do Not Go Gentle into That Good Night" uses light and darkness as symbols for life and death. Light and darkness are classic symbols for these concepts and have been used by artists of all mediums for centuries.
Darkness is often used as a symbol for death, since death is the great unknown for all human beings: just as no one can see in the dark, no one knows for sure what happens after death or if anything happens at all beyond the termination of life. This lack of knowledge gives human beings a primal fear of both the dark and death. In a similar vein, light works as a symbol for life, since in the light, people can see. Both light and life are comforting and familiar. However, light can also be bright, signifying passion. It is this kind of light that the speaker wants his father to evoke.
In the poem, the speaker urges his dying father to "rage against the dying of the light"—that is, to fight his impending death. Just as day must eventually give way to the night, so too, is death inevitable. However, the speaker wants his father to remain in the light as long as possible and to go out fighting instead of passively waiting for his light to die out. Notice how the speaker also uses light imagery to describe the passion of dying men, such as blazing meteors.