"Do Not Go Gentle into That Good Night" was published in 1951 and dedicated to Dylan Thomas's father, who was becoming blind and approaching death. Throughout the poem Thomas implores his father to fight against old age and death, and in the fifth stanza of the poem, Thomas addresses his father's failing eyesight.
In the fifth stanza, Thomas writes that "blind eyes could blaze like meteors and be gay." By this he means that a loss of sight does not equate to a loss of spirit. Even though a man is blind, his eyes may still glow or "blaze" with spirit and passion. It seems here that Thomas is trying to encourage his father by reminding him that neither death nor the loss of his sight can take his spirit from him.
This simile—"blaze like meteors"—evokes an image of a blazing light surrounded by darkness. The darkness here represents the inevitable death that Thomas's father is facing, and the "blazing light" represents an intense brightness of spirit shining in the midst of that darkness, perhaps even all the brighter because of the darkness.
The speaker's tone in this poem is urgent and desperate, indicating how much he loves his father and how afraid he is of losing him. This urgency and desperation is encapsulated nicely in the image of eyes "blaz[ing] like meteors." Meteors light up the sky only for a short time, but their light is brilliant nonetheless. Thomas is aware that his father's spirit can only "blaze" for a short time longer, but he is desperate for it to shine as intensely as it can during that time.