Alexander Pope'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.
362True ease in writing comes from art, not chance,363As those move easiest who have learn'd to dance.364'Tis not enough no harshness gives offence,365The sound must seem an echo to the sense.
It is not enough that a poem should just not be offensive to the ear; it should couple sound with sense-- what it says should be reflected in how it sounds. What follows is a series of beautifully written lines in which the sound of the line matches the meaning of the line. This is best demonstrated by reading the lines aloud. These lines, read aloud, sound like the smoothness of a stream and a soft breeze:
366Soft is the strain when Zephyr gently blows,367And the smooth stream in smoother numbers flows;
The next two lines are just the opposite. The sound of the lines is choppy and rough, something you may not notice by looking but will certainly hear when it is read aloud, and the sound of the words matches the crashing of the waves.
368But when loud surges lash the sounding shore,369The hoarse, rough verse should like the torrent roar.
The next two lines talk about a laborious straining, and the sound of the lines reflect that as much as the words themselves:
370When Ajax strives some rock's vast weight to throw,371The line too labours, and the words move slow;
Just the opposite happens in the next two lines. Here there is a sense of skimming quickly across a surface. Note that although line 373 has only one more word than line 371, it reads much more quickly because of the words Pope chose to convey the meaning.
372Not so, when swift Camilla scours the plain373Flies o'er th' unbending corn, and skims along the main.
The following additional examples demonstrate Pope's assertion that what the poem means (the sense) must be matched by the sound. Read them aloud to see what he wants to teach us.
347And ten low words oft creep in one dull line,
Ten words may be required in some poetic forms, but the wrong ten words create nothing but a monotone. Note the slow movement of a wounded snake in the sound of line 357 and the connection between sound and sense in line 359. (Try to say "languishingly slow" quickly; it is a perfect blend of sound and sense.)
357That, like a wounded snake, drags its slow length along....
Pope's essay is a kind of primer on how to write poetry.359What's roundly smooth, or languishingly slow;