This is no place to add my mite to the confusion that exists on the subject of what is meant by "understanding" a poem.  "Understanding" is something that people more respectable than myself assure me that they burn to apply to everything.  If they look, for example, at a picture, and are in danger of feeling pleasure from it, they either declare that "they don't understand it" or they apply their understandings to some object which, but for their assurances to the contrary, I should have suspected wasn't the picture: in either case, it seems, they feel better for having avoided submitting to the dignity of pleasure.  With a poem, the same sort of difficulty arises.