The theme of the poem is indeed what the author wants to express, as the landlord said, but the theme is not equal to the main content. Let's take Li Bai's Thoughts on a Quiet Night as an example. Its main purpose is to miss home, and its main content is to look down at the moonlight in front of the bed and miss home. The main purpose of Wanglushan Waterfall is to praise the great rivers and mountains of the motherland, and its main content is to show that the waterfall has a large drop and is magnificent.
Can you understand? I hope it helps you.