The speaker opens with a command, addressing an unknown listener, to resist dying peacefully and instead to fight hard against death, despite its inevitability, using night and day as metaphors for death and life. He states that “wise men” know that death is ultimately right, but that they nonetheless combat it because they haven’t left enough of a mark on the world. The speaker continues to use natural imagery in this second stanza, likening the failure of words to leave a mark on the world to an inability to "fork," or redirect, lightning.
In the third stanza, the speaker adds that “good men,” too, stand against death for similar reasons. Despite their virtue, their deeds remain "frail" and haven't stood out sufficiently as a strong, massive wave in a calm bay would. These men, too, fight death in hopes of leaving a lasting legacy. The fourth stanza continues this theme as the speaker discusses “wild men,” who spend their lives on futile adventures, failing to appreciate how short life is until they face death, which they refuse to meet calmly. Again, the speaker uses natural imagery, comparing wild men's adventures to the excitement yet ultimate fruitlessness of "ca[tching] and s[inging] the sun."
“Grave men,” he continues, realize that they can die dramatically—“blaze like meteors”—by refusing to yield to death. In their old age, they have a clarity that escaped them when they were younger, now able to "see with blinding sight," or understand things with piercing lucidity. Finally, the speaker reveals that he is addressing his father, who is dying, and urges him to show emotion, like men he has just described.
In the first stanza, Thomas uses day and night as an extended metaphor for life and death, urging people to resist death courageously rather than simply accepting it. By using this metaphor, he presents life and death as part of the endless natural cycle of time, which began long before our lives and will go on long after them, just as day and night are a part of it. This gives death an impersonal feel: if everyone and everything dies, there's little that's special or notable about one death. This generic conception of death is what Thomas's characters in the poem are fighting against, striving to give their deaths individual significance.
With the similar-sounding words “rave” and “rage,” Thomas emphasizes anger and passionate intensity in the face of death. Though he acknowledges that death may be “right”—after all, everyone dies eventually as part of the natural process discussed above—at the beginning of the second stanza, he writes that “wise men” refuse to accept it, because they haven’t yet left enough of an impact on the world. They’ve “forked no lightning,” or failed to create a big burst of light—here a symbol for life—that would give them a legacy.
In the third stanza, the speaker insists that “good men” similarly see their actions as “frail” and long to stand out more, as a wave does in a calm bay. The critic Rushworth M. Kidder suggests that "green bay" may be an allusion to Psalms 37:35: “I have seen the wicked in great power, and spreading himself like a green bay tree.” Following this interpretation, the good men's frail but virtuous deeds would have been more remarkable when seen in contrast to the evil of the “green bay.” Without any great evil to fight against, these men's virtues are less noteworthy.
The fourth stanza continues the now-familiar pattern of the poem, with the speaker describing “wild men” who "caught and sang the sun in flight,” or in other words, celebrated the world around them through bold actions and feats, and belatedly realized the brevity of life. By returning to the sky as a source of imagery, Thomas reemphasizes the central day/night metaphor of the poem.
The stanza may also allude to the Greek myth of Icarus, who flew too close to the sun, melting the wings his father had crafted for him and causing him to plummet to the ground and his death. This myth is often understood as a warning against hubris, or excessive pride. With that allusion in mind, the lines indicate that the wild men were too proud to realize that death would eventually befall them too, no matter how grand their adventures.
The speaker continues in the fifth stanza, discussing "grave men” (who are grave in the sense of being serious, but also in the sense of being near death) who see fixedly with piercing sight that they must fight death as well, choosing to go out “like meteors,” imagery that again returns to the sky. Thomas's employment of the image of meteors also recalls the impersonal vastness of the cycle of life and death: meteors, too, are so immense, scalding, and fast that it's difficult to comprehend them. Like night and day as well as lightning, they're also transient, appearing to us for only moments in the night sky.
In the final stanza, the speaker reveals that he has been addressing his father, which gives the poem a personal significance it previously lacked. The speaker again underscores the poem’s message, urging his father to show any sort of emotion in the face of death. The "sad height" may be an allusion to the Bible's valley of the shadow of death, which appears in Psalm 23. The phrase is often misquoted as simply the "valley of death," but if death is casting its shadow on the valley, it must be above the valley, like the father on the "sad height" of the mortal realm. Despite the anguish that this expression of grief and fear would cause him, the speaker longs for his father to cry at his impending death, because it would show that his father still has vitality and dignity. It's hard to see our parents, especially traditionally stoic fathers, cry, but it reminds us of the full range of their humanity and the vulnerability that comes with that humanity.