Night represents death, or dying. The poem is written about Dylan Thomas's father, who is dying at the time, and the opening line implores him to "not go gentle into that good night", or to not die without a strong fight. Further evidence comes in fourth line where Thomas states, "Though wise men at their end know dark is right." At their end means at the end of their lives, and the dark refers to the night, or death.
Light in the poem represents life, or life's force and vitality. Dylan wants his father to "rage against the dying of the light", or to fight against his life draining from him. Various forms of light are discussed in the poem, all representing different forms and strengths of living. You have men whose "words had forked no lightning", the light there representing the activating, and powerful words that men might have used in their lifetimes; there is "how bright their frail deeds" might have been in life, so men regretting not living more "brightly", or powerfully; and men "who caught and sang the sun in flight" representing those powerful figures who seem to blaze with light in all that they do. For all of these examples, light represents life, and the different things men do or do not do in their lives. And in the end, Dylan wants all men, regretful or not, to rage against the dying of the light.
Light and night are opposites, just as death and life are, so they are apt descriptions to use in this poem.