An annotation comments on or explains a text or a part of a text (Merriam-Webster). In this case, the goal is to comment on or explain the following lines from "Ode to a Nightingale" by John Keats:
Thou wast not born for death, immortal Bird!
No hungry generations tread thee down . . .
These lines come in the penultimate or second-to-last stanza in this poem. The narrator is daydreaming about death. He notes in the stanza before this one how beautiful it would be to die at "midnight with no pain" while hearing the meltingly lovely notes of the nightingale. In other words, the poet would like to drift off into death with that beautiful and ecstatic song filling his ears and mind.
At the start of the next stanza, the narrator says that the bird is "immortal" (though of course the individual bird he hears will die) because its beautiful music has lasted through the ages. The nightingale has not been beaten down by the hunger or pain it has witnessed in people's lives, but rather has always kept up its joyful song. The narrator then goes on to imagine other times and places where the song of the nightingale gives comfort. He specifically thinks of the biblical book of Ruth. The young Ruth was a hungry stranger in a strange land, having followed her widowed mother-in-law Naomi to a new home. Ruth was filled with sadness and uncertainty, yet the narrator imagines the nightingale giving her comfort:
Perhaps the self-same song that found a path
Through the sad heart of Ruth, when, sick for home,
She stood in tears amid the alien corn . . .
It's important, too, to note that this poem was written within two years of Keats's early death from consumption (what we today call tuberculosis). At the time, there was no cure for it, and it usually meant a slow and painful death. Keats would have had death and suffering on his mind as he wrote this verse.