In "Ode to a Nightingale," the speaker discusses how the nightingale's song is immune to sorrow and the pains of life. The speaker gives the nightingale's song an immortal quality; since it is so joyful, the nightingale presumably never thinks of death and is therefore eternal. The speaker supposes that people throughout history have heard the song and had similar reactions.
But, even though the nightingale's song has this immortal, eternal aspect, it can only give temporary relief. The speaker alludes to this in the second and third stanzas, indicating that he would have to be literally or metaphorically drunk enough to forget sorrow and mortality because, for the conscious person, "Where but to think is to be full of sorrow." By living vicariously through the nightingale's song, the speaker leaves and forgets his worries.
In the last stanza, the speaker says that his vicarious living through the nightingale's song is/was temporary. The word "forlorn" snaps him out of his reverie and the nightingale's song fades:
Was it a vision, or a waking dream?
Fled is that music:--do I wake or sleep?