Dylan Thomas's poem "Do Not Go Gentle into That Good Night" is a villanelle, which means that it has a strict form. This particular form has two refrains, or repeated lines. The first refrain says,
Do not go gentle into that good night.
The second refrain insists,
Rage, rage against the dying of the light.
This poem discusses death; specifically, the speaker addresses his father's impending death. This is seen in the final stanza as he addresses his father directly:
And you, my father, there on the sad height.
In this poem, the speaker asks his father not to accept death passively. Instead, he wants his father to combat death and to live as long and fully as he can. The dying of the light may refer to the end of the day, when night comes. This nightfall likely symbolizes death. It could also refer to the darkness that comes when a person succumbs to death and can no longer see. Their eyes close; there is no more light for them. The speaker of the poem wants his dying father to fight darkness and death, to "rage against" it, and to keep the light, meaning his life, as long as possible.
Notice the meaning of the word rage. Rage, as a verb, means to be angry, furious, or incensed against something. The intensity and meaning of the phrase, and the poem overall, changes when "rage" is replaced with other, similar words. If the poet instead chose to write "Be angry about the dying of the light," the line wouldn't pack nearly the same intensity, affecting the poem's meaning and mood altogether.
Thomas argues that life is more than merely breathing, eating, and sleeping. By telling his father to rage against death, the speaker urges him to live an active life, full of significance, until his dying day.