Step 1: Poet and poem.
The poem is written by John Keats, a romantic poet.
Step 2: Central theme.
Keats describes how beautiful things in nature, like trees, flowers, streams, and the sun, give humans eternal joy and inspiration.
Step 3: Message.
Beauty is a source of hope and comfort, helping people overcome sadness and negativity.
Step 4: Conclusion.
Thus, the central idea is that beauty never fades; it brings lasting happiness to human life.
Final Answer: \[ \boxed{\text{Beauty provides everlasting joy and relieves human suffering.}} \]