Give an annotation on this quote: "Thou wast not born for death, immortal Bird! No hungry generations tread thee down."

This is a quotation from the seventh stanza of John Keats's "Ode to a Nightingale." As the title indicates, this poem is addressed directly to the nightingale. The type of direct address we see in this line, "Thou wast not . . ." is a rhetorical device called apostrophe, wherein the speaker is addressing a person or thing who is not actively present or engaged. The nightingale (being, in reality, only a bird) cannot understand what the poet is saying to it, but the speaker has personified—indeed, deified—the bird, and therefore it makes sense for him to address the nightingale here in an accordingly respectful way.
Keats's use of language here is also connected to the way he portrays the bird as "immortal." Since he lived in the early 19th century, Keats would not have used constructions like "thou wast" in his everyday speech. He chooses deliberately archaic language, the sort of language we might associate with the King James Bible, which has the effect of emphasizing the sense that the bird is being addressed as if it were an immortal being. For "generations," the speaker suggests, the bird has existed without having suffered the "tread" of anything else: like a god, the nightingale is constant, unchanging and ageless.
In juxtaposing the words "born" and "death," Keats is also emphasizing the supposed immortality of the nightingale, which he suggests has existed for centuries and sung the same song the ancients heard. We are, of course, all "born for death," but in conjunction with the word "immortal," this line establishes a semantic field of immortality which seems to set the bird outside of the natural order of things. The nightingale, Keats is saying, was not born to live ephemerally and be trodden down by the "hungry," or needy, generations—he was not born to be consumed or destroyed. Instead, he is a changeless and constant being which seems to connect the speaker to those who heard the nightingale sing many centuries earlier.