Of beechen green, and shadows numberless, Singest of summer in full-throated ease. (lines 9-10)
The nightingale is in its prime, and the natural things around it are in the peak of their goodness. Keats's poetry often focuses on the transience of the seasons, and especially his odes, like "To Autumn" and "Ode on a Grecian Urn," which describe fall and springtime, respectively. This ode captures the glory of summer.
Where beauty cannot keep her lustrous eyes, Or new Love pine at them beyond to-morrow. (lines 29-30)
Out with the old, in with the new. For the speaker, this adage captures all that's wrong with the world. People never have time to savor the good things. Beauty and love always seem immortal, but after "to-morrow," they might be gone forever. The poem seeks an escape from time.
But, in embalmèd darkness, guess each sweet Wherewith the seasonable month endows (lines 43-44)
The nightingale's world is dark, but it still captures the peak of summertime. Summer is "seasonable" because all the plants and flowers (those "sweets" he refers to) are out in full force. But this world is not transient; it is "embalmed" with the scents, like a tomb. Death – now there's one way to escape time.
Adieu! adieu! thy plaintive anthem fades Past the near meadows, over the still stream, (lines 75-76)
When the spell of his vision is broken, the speaker must leave the fragrant, frozen midnight world of the nightingale and return to the real world, where it's just another sunny day. The flight of the nightingale is the height of nature's transience. Just when the speaker was starting to get into a groove, too.
Fled is that music:—do I wake or sleep? (line 80)
Yes, the end of the poem kind of plays that old trick from bad TV and movies – he has been dreaming the whole time! It only takes him two lines to forget all about the reality of the nightingale.