It has always seemed to me that the poet's intention was to have "Do Not Go Gentle Into That Good Night" read like a rondo form in classical music, in symphonies, concertos, and sonatas. This form seems appropriate since the rondo movement always comes at the end of the three or four movements of the musical piece. In a rondo the same musical phrases are repeated over and over but with slight variations and in different placements. In Thomas's poem the phrase "Do not go gentle into that good night" keeps reappearing. In the first stanza it is in the first line. Then in the second stanza it pops up in the last line. The other phrase that keeps returning is "Rage, rage against the dying of the light." It appears as the last line in the first stanza and again as the last lines in the third and fifth stanzas. Finally both these phrases end the poem when they appear together in the sixth stanza.
Do not go gentle into that good night. Rage, rage against the dying of the light.
I believe the thought content, or "meaning," is secondary to the musical quality of the poem. Like a rondo it is a sort of simple finale to a serious composition. Presumably the other movements were represented by the poet's father's life itself. The distinguishing quality of any rondo is repetition. Notice how the last line of every stanza is either "Rage, rage against the dying of the light" or "Do not go gentle into that good night." The two lines alternate regularly until the last two stanzas, which both end with "Rage, rage against the dying of the light."
The poet knows that he cannot really give his father any practical advice about dying. His father may already be dead by the time he writes his poem. He is just paying his father a tribute by creating a poem in his honor. The fact that the poet keeps repeating the same two phrases, "Do not go gentle into that good night," and "Rage, rage against the dying of the light," suggests that he really doesn't have much more to say because there is no way of changing the reality that his father is either dead or dying. In addition to being a tribute to his father, the threnody is intended as a means of easing the speaker's own feelings. The repetition is futile as a means of conveying information but is perhaps helpful in mourning the dead or dying man.