Please explicate the following lines from Keats' "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!15
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: 20
In these lines the speaker, Keats himself, is only expressing the wish that he had something to drink and that he could get thoroughly intoxicated. Keats was a young man, but he was developing a strong liking for intoxicants. Many creative writers have a liking for alcohol and drugs which can be dangerous, as I believe Emerson noted in his essay "The Poet."
In the beginning of this poem Keats mentions hemlock and an opiate, which would probably be a mixture of opium and alcohol. Later in the poem he thinks of the musk rose as "the murmurous haunt of flies on summer eves"--in other words, as a sort of pub where the flies hang out and get drunk on the nectar the flower produces.
Keats was troubled by thoughts of death. Several members of his family had died, and he himself expected to die of consumption. A number of his poems deal with thoughts of death, including "Bright star, were I as steadfast as thou art," and "When I have fears that I may cease to be." There was no cure for his disease, so he tried to escape from thinking about it by at least two ways--drinking and writing poetry. In "Ode to a Nightingale," which he wrote while listening to one of these melodious birds late at night in a friend's garden, he is simply saying that he would like to get drunk and escape from his morbid thoughts--but he just doesn't have anything to drink. So he decides that he will try to escape in his imagination, which he calls "the viewless [invisible] wings of poesy."
Keats excelled in sensuous descriptions. He was not a deep thinker, but had a powerful visual imagination. The lines quoted in your question are nothing but a description of the kind of wine he wishes he had. He would like a whole beaker (about a quart). It would be a red wine from the south of France. No such wine is produced in England; it would have to come from the wine country of Europe.
The best lines in this stanza are:
O for a beaker full of the warm South!
Full o the true, the blushful Hippocrene,
With beaded bubbles winking at the brim
And purple-stained mouth.
The warm south held a strong attraction for Keats. He felt that the warmth of southern France or Italy might heal him, or at least make him suffer less than he did in cold, damp England. He actually went to Itay to try to recover his health. He died there and is buried beside another great English poet, Percy Bysshe Shelley, with whom he is often compared. The Merriam-Webster online dictionary gives the followiing definition of Hippocrene:
: a fountain on Mount Helicon sacred to the Muses and believed to be a source of poetic inspiration
Evidently Keats like to drink for poetic inspiration as well as for escape from his thoughts of death.
Everyone must have noticed how the little bubbles collect at the top of a glass of wine and cling to the rim and could be said to be strung together like beads. These little bubbles pop and might be said to be winking. A person's tongue does become purple-stained when drinking red wine. This type of imagery is characteristic of Keats' poetry and is the best thing about most of it. The entire poem is full of vivid visual descriptions that will take the sensitive reader along with the poet into the world of the immortal nightingale. F. Scott Fitzgerald loved Keats and borrowed the beautiful phrase "tender is the night" from the fourth stanza for the title of his best novel.