A poem's theme is its central idea or insight about life, human nature, love, loss, memory, time, or another important subject. It is not just the topic of the poem, but what the poem suggests about that topic. A poem may have more than one theme, and the theme is often revealed through images, events, tone, and repeated details rather than stated outright.
Example
A poem about winter may use these lines to suggest a theme of endurance:
The frozen path bends underfoot,
but still the creek keeps moving
under its sheet of ice.
[n] a unifying idea that is a recurrent element in a literary or artistic work; "it was the usual `boy gets girl' theme"
[n] (linguistics) the form of a word after all affixes are removed; "thematic vowels are part of the stem"
[n] an essay (especially one written as an assignment); "he got an A on his composition"
[n] the subject matter of a conversation or discussion; "he didn't want to discuss that subject"; "it was a very sensitive topic"; "his letters were always on the theme of love"
[n] (music) melodic subject of a musical composition; "the theme is announced in the first measures"; "the accompanist picked up the idea and elaborated it"