Describe the following lines in "Ode to a Nightingale."
O for a draught of vintage! that hath been
Cool'd a long age in the deep-delvèd earth,
Tasting of Flora and the country-green,
Dance, and Provençal song, and sunburnt mirth!
O for a beaker full of the warm South!
Full of the true, the blushful Hippocrene,
With beaded bubbles winking at the brim,
And purple-stainèd mouth;
That I might drink, and leave the world unseen,
And with thee fade away into the forest dim: (11-20)
Keats, the narrator, is trying to become like the nightingale. He considers the bird immortal because this nightingale reminds him of nightingales he has read of throughout the history of literature and poetry. Since this bird is linked to all other nightingales in history, it is as if the nightingale (and all nightingales) is immortal.
Since Keats cannot physically become the nightingale, he tries to create imagery and metaphors that will verbally take him there. In other words, he is trying to write his way there. The narrator is clearly trying to be like the nightingale, but why? He knows the nightingale is not really immortal. But perhaps the nightingale believes he (the nightingale) is immortal.
The narrator admires the beauty of the nightingale's song and the beauty of nature in general. Keats often writes of his awe for nature and the tragedy of being aware that he will die someday. When he dies, he will no longer be able to appreciate nature's beauty.
So, the narrator admires the nightingale's natural beauty but (and this is the key) he also envies the fact that the nightingale does not have to face the constant conscious awareness of mortality (death).
Keats, the narrator, a human, does have to live with thoughts of his own mortality. In the third stanza, he writes about this:
Fade far away, dissolve, and quite forget
What thou amongst the leaves has 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; (21-25)
In the stanza (2nd) which you asked about, the narrator is asking for a wine (vintage) so that he may become (metaphorically) drunk enough to forget that he is mortal. Thus, he can become more like the nightingale: full of beauty but no longer plagued by thoughts of death. He is actually writing about being drunk enough to forget about the fact of death. Then he can enjoy nature without the constant fear that he will someday lose the ability to enjoy nature.
Also note that Hippocrene refers to a fountain in Greek mythology which could provide poetic inspiration when consumed.