The first aspect is knowing the language of that particular time period. Of course we cannot assume that all words mean the same thing now as they did when certain authors used them.
Words change meaning over time.
Knowing a general history of where and when the author wrote the poem is extremely important in understanding the work. I disagree with Gerlad Graff in that you have to know the history and read the critical analysis of the work in order to understand it. In most cases you are reading what the critcs are interprating and not necessarily what the author had intended. After reading what critcs have to say it is almost impossible to form your pwn opinion without feeling that you are wrong in disagreeing with them. Whereas if you know the history surrounding the work you are better able to get in to the frame of mind of tha author.