Hello,

The main idea is about the main theme of the poetry, it deals with the big picture of the piece. You can go through the below mentioned links to read about 'The study of the poetry'.

https://www.poetryfoundation.org/articles/69374/the-study-of-poetry

https://englishsummary.com/study-poetry-matthew-arnold/

http://learningliteratureoverhere.blogspot.com/2016/08/a-study-of-poetry-matthew-arnold.html?m=1

Hope this helps, Good luck.