what poet saying hereThrough the sad heart of Ruth, when, sick for home, She stood in tears amid the alien corn; The same that ofttimes hath...

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.