In the poem "Do Not Go Gentle into That Good Night," the speaker desperately implores a loved one to fight against old age and death. The loved one in this instance is likely the poet's father, who, at the time the poem was published, was close to death, dying one year later.
The line "Do not go gentle into that good night" is repeated four times in the poem, as is the line "Rage, rage against the dying of the light." The fact that these lines are repeated so often emphasizes how desperate the speaker is for his father to fight for his life. This desperation is also emphasized by the fact that the poem begins with one of these lines and concludes with the other, meaning that the speaker's desperation is the first and last impression of the poem. Both lines are also written as imperative sentences, or orders, furthering the impression that the speaker is so desperate that he refuses to allow his father to do anything but rage and fight against his death.
The "good night" is of course a euphemistic reference to death. The fact that the speaker refers to death in this euphemistic way suggests that he is trying to comfort himself with the idea that when death does take his father, it will at least be gentle and comforting. It may even offer his father some respite from the pain and suffering of his life. The repetition of the "good night" euphemism emphasizes how much comfort the speaker needs to give himself and thus how desperate and upset he feels about his father's impending death.