To understand "Tithonus" by Alfred, Lord Tennyson, one needs to know the myth of Tithonus and Aurora. In the Greek legend, Aurora (or Eos), goddess of the dawn, fell in love with a man named Tithonus. Aurora, being immortal, asked Zeus to grant her husband immortality, which he did. However, Aurora forgot to mention that she wanted Tithonus to have eternal youth as well, so Tithonus's body aged and withered, but he was unable to die.
The poem is written with Tithonus as the speaker. He says to Aurora, "Me only cruel immortality consumes: I wither slowly in thine arms." Stanza 1 is a lament by Tithonus about his state. Stanza 2 recounts how Aurora is so freely granted this request for immortality and how Tithonus aged. Now, instead of being more like Aurora, he is even more her opposite. She is immortal youth, but he is immortal age. He asks her to take back her gift and let him go.
Stanza 3 describes the beauty of the sunrise, but Stanza 4 notes that as the day progresses and turns to evening, Aurora remains silent, not answering his request that she take back her gift. Stanza 6 suggests that she does not answer because she does not want him to know the hard truth, namely, that gods cannot take back the gifts they have granted.
In Stanza 7, Tithonus remembers the days of his youth, when he and Aurora could enjoy the pleasures of being young together. The final stanza contrasts the warm love he shared with Aurora in his youth to the coldness of his existence now that he has aged. He longs to have the "power to die" and envies those who are already in their graves. He again asks to be released from immortality and assures Aurora that she will still see him when she sees his grave.