Keats opens his "Ode to a Nightingale " by wishing he had a whole beaker of wine, so that he could get drunk and escape from the real world where there is so much unhappiness and where so many people like himself are haunted by the fear of death....
Keats opens his "Ode to a Nightingale" by wishing he had a whole beaker of wine, so that he could get drunk and escape from the real world where there is so much unhappiness and where so many people like himself are haunted by the fear of death. Then, since he has no wine available, he decides to try to escape in his imagination to the world where the hidden immortal nightingale is singing in the shrubbery.
Everything he says in the quoted lines has to do with the world he imagines inhabited by the nightingale. It is dark in there because it is nighttime and there is thick shrubbery. So he cannot see what flowers are at his feet or see what kind of incense there is on the surrounding boughs, although he can smell everything in his imagination and guess which plants and flowers are producing these smells. By "embalmed darkness" he means pleasantly scented darkness, and the accent over the last syllable of "embalmed" indicates that this should be pronounced as three syllables.
So Keats guesses that he and the singing bird are hidden among
The grass, the thicket, and the fruit-tree wild; 45 White hawthorn, and the pastoral eglantine; Fast-fading violets cover'd up in leaves; And mid-May's eldest child, The coming musk-rose, full of dewy wine, The murmurous haunt of flies on summer eves
Eglantine is also called "sweetbriar." "Fast-fading violets cover'd up in leaves" is an interesting line because it breaks with the dominant iambic pentameter and is actually syncopated. Perhaps this is meant to emphasize their being fast-fading. The musk-rose has musk-scented white flowers. Keats is still craving wine and imagines the musk-rose being filled with nectar which attracts flies to gather around it the way men gather around the bar in a pub and talk in murmurs.
Those two lines--about the fast-fading violets and the murmurous haunt of flies--show Keats's brilliant imagination, the quality which made him one of England's greatest poets. He died of tuberculosis when he was only twenty-six years old, a great loss to English literature.