Keats is stating that, in his opinion, nightingales have been alive and present throughout human history. "Thou wast not born for death, immortal Bird!" He is supporting this opinion with various theories or demonstrations of a nightingale's presence at places and times in the past.
Keats suggests that the call of a nightingale was heard "in ancient days by emperor," implying that nightingales had lived in the Roman Empire and other long-ago empires.
He refers to Ruth, the Biblical Moabite widow who follows her widowed mother-in-law back to the mother-in-law's homeland near Bethlehem, harvests "the alien corn" from the fields to allow them to live, and eventually remarries and becomes the great-grandmother of King David, the greatest of the Israelite kings.
Keats places the call of the nightingale throughout the world of fantasy, referring to "magic casements" and "faery lands."