's "An Essay on Criticism
" is a discussion and critique of the art of poetry, and poetry readers, of his day. The lines to which you refer are in Part Two of the poem and talks about the right and wrong way to write poetry. It is a masterful display of the importance of both sound and sense in poetry.
Pope criticizes those who judge poetry based simply on the fact that it precisely follows a certain form ("by numbers judge a poet's song") but not on what it says or means ("to please their ear / Not mend their minds"). He criticizes poets who write "expected rhymes," predictable because they follow a form rather than express a new or innovative thought.
350Where'er you find "the cooling western breeze",
351In the next line, it "whispers through the trees":
352If "crystal streams with pleasing murmurs creep",
353The reader's threaten'd (not in vain) with "sleep".
Instead of such mindless conformity and predictability, Pope reveals what makes an effective poem.
(The entire section contains 586 words.)