To Byron
Pity's plaintive lute
Keats personifies Pity as a musician playing a mournful stringed instrument—Byron is positioned as the one who overhears and preserves these tones in his poetry.
dying swan
Classical reference to the belief that swans sing most beautifully just before death. Keats calls Byron a dying swan still producing enchanting music.
pleasing woe
The oxymoron at the poem's end—Byron's achievement is making sorrow pleasurable through art. This is what Romantic poetry claims to do.