What is the explanation of the third stanza of "Ode to a Nightingale."

Fade far away, dissolve, and quite forget
What thou among the leaves hast never known,
The weariness, the fever, and the fret
Here, where men sit and hear each other groan;
Where palsy shakes a few, sad, last gray hairs,
Where youth grows pale, and spectre-thin, and dies;
Where but to think is to be full of sorrow
And leaden-eyed despairs,
Where Beauty cannot keep her lustrous eyes,
Or new Love pine at them beyond to-morrow.

This third stanza is a continuation of the thoughts expressed in the first two stanzas. Keats was haunted by his fears of death. He had tuberculosis and died of the disease in Italy when he was only twenty-six--a great loss to English literature. Percy Shelley commemorated Keats as Adonais in his poem of that title. 

Keats obviously wants to escape from his melancholy thoughts about his mortality. Evidently he was becoming overly fond of wine as a means of escape. He dreaded death because it would put an end to his creative work when it had scarcely begun. He was in love with a girl named Fanny Brawne but could not marry her because he expected to die. She is probably the Beauty he has in mind where he says:

Where Beauty cannot keep her lustrous eyes,
Nor new love pine at them beyond to-morrow.

And he is the youth he has in mind in the line:

Where youth grows pale, and spectre-thin, and dies;

The lines that immediately follow are especially revealing:

Where but to think is to be full of sorrow
And leaden-eyed despairs,

Keats does not want to think. He wants to forget. He would prefer complete oblivion, but since that oblivion through the imaginary beaker of wine is not available, he will try to escape "on the viewless wings of poesy," as he says in the fourth stanza. That is, he will escape into pure fantasy. He succeeds in doing so--but only temporarily. In the last stanza he says:

Forlorn! the very word is like a bell
To toll me back from thee to my sole self!
Adieu! the fancy cannot cheat so well
As she is fam'd to do, deceiving elf.

What appeals to Keats about the nightingale is that it seems to be immortal. It doesn't have to worry about death, as he does. It seems immortal because the bird always looks like the same bird and always sings the same notes. In his imagination he escapes into that bird's world. At first he finds himself in a fantasy world under the bushes where the bird customarily nests. He describes that little world with its scents and flowers. But then he travels back in time to when Ruth of the Old Testament heard that very same bird singing that very same song "amid the alien corn." And he travels beyond time into "faery lands forlorn" before he is called back to reality by the word "forlorn" which reminds him of his forlorn condition. 

The third stanza is a sort of prelude to Keat's taking off into his imagination to escape from his thoughts of death. In the next stanza he will seem to take flight.

Away! away! for I will fly to thee,
Not charioted by Bacchus and his pards,
But on the viewless wings of Poesy...

Where