Dylan Thomas, in this poem, is probably sitting at the deathbed of his father--since it is a poem about how he wants his father to greet dying and death, it is implied that he is watching his father struggle with life. He wants his father to greet death with strength, and to die fighting until the very end. This sentiment is found in his repeated admonition to "rage, rage against the dying of the light." In that line, the light is life, and that life is dying away. Thomas tells him to be show rage and anger, to fight against his life fading away. He wants him to leave his life strong, and proud, and not to just give up and fade away, to give into death weakly without a fight.
He asserts that ALL men, no matter what their lives were like, "wise men," "good men," "wild men," "grave men," and similarly, "you, my father" have all raged against death, and not gone "gently into that good night." He has loved his father, and it is distressing him to see his father close to dying, so, he at least wants to see his father as he knew him in life, strong and fighting. It's an intensely emotional poem, filled with a battle-cry to those moving on in this world, to move on with strength and pride.
I hope that those thoughts help; good luck!