Do not go gentle into that good night, Old age should burn and rave at close of day; Rage, rage against the dying of the light.
The speaker addresses an unknown listener, telling him not to "go gentle into that good night."
At first this is a puzzling metaphor but, by the end of line 3, we realize that the speaker is using night as a metaphor for death: the span of one day could represent a man's lifetime, which makes the sunset his approaching demise.
"That good night" is renamed at the end of line 2 as the "close of day," and at the end of line 3 as "the dying of the light." It's probably not an accident that the metaphor for death keeps getting repeated at the end of the lines, either. Or that the two rhyming words that begin the poem are "night" and "day."
So what does the speaker want to tell us about death? Well, he thinks that old men shouldn't die peacefully or just slip easily away from this life. Instead, they should "burn and rave," struggling with a fiery intensity.
The word "rave" in line 2 connects with the repeated "rage" at the beginning of line 3, uniting anger, power, madness, and frustration in a whirlwind of emotion. Oh, yeah, it's going to be one of those poems. Get ready to feel.