The author is referencing the story of Ruth in the Bible where she refuses to leave her mother-in-law and works in the fields. He is implying that Ruth heard the nightingale while she was working in the fields and it made her break down in tears.
The author then makes reference to another time the nightingale was heard singing but it's some magical place.
The casement is referencing a window with a hinge. We are led to believe that the nightingale's singing is o magical that it opens a window on a ship and then he flies out into the open air over the ocean. But, again, this is a majical ocean, not the ordinary ocean you and I know. This is where Keats puts a huge emphasis on fantasy. That is why it is so hard to understand.