Metaphors in Poems

What is a metaphor? Please define. Then identify at least three metaphors from the poems assigned (ones to "pay attention to") to explain how these metaphors function to make the poems more effective. Two hundred (200) words or more in 12 point.
It would be helpful to view the following video, but not required.
Walt Whitman Video: Walt Whitman, An American Original
https://sunysuffolk(dot)kanopystreaming(dot)com/video/walt-whitman-american-original
You must post your answer by midnight Friday. The assignment will close at the deadline and no late submissions are accepted.
Poems assigned are:
“Shall I compare thee to a summer’s day?” By William Shakespeare (780-781); “A Noiseless Patient Spider” (Internet); Introduction to Walt Whitman (A Concise Biography: Video); Poem to read: “When I heard the Learn’d Astronomer” by Walt Whitman (1069); “One’s-Self I sing” by Walt Whitman (1069); “Cross Brooklyn Ferry” (online);