The main idea of a poem is also known as

A theme (also known as a motif) is the main driving idea behind a poem. A theme or motif is not a summary of the poem, or a detail from the poem, but rather the emotion or motivation behind the poem. The theme might be "unrequited love" or "the power of traditions," but it wouldn't be "putting up walls between property lines," or anything else that specific.

The answer is subject read page 8 of the study unit