"Do Not Go Gentle into That Good Night" laments the necessity and inevitability of death, encouraging the aged to rebel against their fate. The poem suggests that (to use an old cliché) we should leave this world the way we came in – kicking and screaming, holding on to life for all we're worth.
Although "Do Not Go Gentle into That Good Night" depicts death as inevitable, the speaker suggests that people can redeem themselves by bravely fighting against the odds, resisting death to the last.
The speaker's decision to use sunset as a metaphor for death implies that there is a redemption or reawakening after death, since every sunset must be followed eventually by a sunrise.