Here's why:
* Metaphor: A metaphor is a figure of speech that makes a comparison between two unlike things without using words like "like" or "as". In this case, "a page of prancing poetry" is being compared to "coursers" (horses).
* The comparison: The poet is suggesting that a page of poetry is as exciting and vibrant as a horse racing through a field.
The phrase uses a creative and imaginative comparison to highlight the energy and movement found in poetry.