To fully understand poetry, we must first be fluent with its meter, rhyme and figures of speech, then ask two questions: 1) How artfully has the objective of the poem been rendered and 2) How important is that objective? Question 1 rates the poem's perfection; question 2 rates its importance. And once these questions have been answered, determining the poem's greatness becomes a relatively simple matter.
If the poem's score for perfection is plotted on the horizontal of a graph and its importance is plotted on the vertical, then calculating the total area of the poem yields the measure of its greatness.
A sonnet by Byron might score high on the vertical but only average on the horizontal. A Shakespearean sonnet, on the other hand, would score high both horizontally and vertically, yielding a massive total area, thereby revealing the poem to be truly great. As you proceed through the poetry in this book, practice this rating method. As your ability to evaluate poems in this matter grows, so will, so will your enjoyment and understanding of poetry.
And I deliberately leave the context to last ...