In the poem "Ode to a Nightingale," Keats brings to the reader's attention two very different worlds.
The first is that of the nightingale. With beautiful imagery the poet creates the world this songbird inhabits, with visions of lush plants, sweet smells, soft breezes and, most importantly, release from the pain of the second place he describes.
This other world is the one he, and all people, inhabit during different times of their lives. Keats speaks to the beauty of youth that fades and the passion of love that dies. He refers to aging and the groans of pain, and finally of death.
He wishes to be transported to the world of the nightingale, one that has existed for "emperor and clown in ancient times;" a voice that does not know death. Keats alludes to the song being sung when the Biblical Ruth, missing home, stood in the corn fields of others, "aliens"—not in the company of her own people.
As Keats writes, he wishes to be a part of the nightingale's world that knows the beauties of nature in ways he can only observe. Life, even as he dreams of this other world, comes and calls him back to the reality of his existence...the existence of all people: we live, we love, we suffer and we die, but he says the song of the nightingale lives on forever, throughout the passage of time.
As an aside, death would have been something Keats thought about often. This poem was published in 1818, and Keats died in 1821. He was unwell for a very long time, and died of tuberculosis when he was only twenty-six, having also lost his mother and brother to the same disease; the pain of these losses, his own personal illness, as well as early experiences in his life while working at a hospital, make up the sad and lonely descriptions he shares in this poem.