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Alexander Pope's 'An Essay on Criticism', lines 337–349:

But most by Numbers judge a Poet's Song,

And smooth or rough, with them, is right or wrong;

In the bright Muse tho' thousand Charms conspire,

Her Voice is all these tuneful Fools admire,

Who haunt Parnassus but to please their Ear,

Not mend their Minds; as some to Church repair,

Not for the Doctrine, but the Musick there.

These Equal Syllables alone require,

Tho' oft the Ear the open Vowels tire,
While Expletives their feeble Aid do join,

And ten low Words oft creep in one dull Line,

While they ring round the same unvary'd Chimes,

With sure Returns of still expected Rhymes.

I don't understand the line in bold. Can you help me?

Pope is using the word ‘expletive’ in this sense:

A word or phrase that fills out a sentence or metrical line without adding anything to the sense; a word or phrase serving as a grammatical place-filler.

_{Oxford English Dictionary. ‘Expletive’, n. sense B.1.a.}

(This sense later evolved into the modern sense of the word, “an emphatic exclamation with which a person fills his or her speech; (hence) an oath, curse, or swear word, a profanity.”)

An example of such an expletive is the word ‘do’ in this very line, which has no function other than filling a gap in the rhythm. Pope of course put this here deliberately in order to illustrate the problem (just as in the next line, which complains about the monotony of ‘ten low words’ in a line, there are ten single-syllable words).

In this passage Pope is accusing ‘most’ critics of judging poetry by counting the number of syllables in each line, without regard to other qualities of verse. These critics don’t care how the poet filled the line, so long as he was able to count to ten.

Pope is talking about how most people just care about how a poem sounds ("Who haunt Parnassus but to please their Ear") and not what it actually says ("Not mend their minds"). In particular he notes how "ten low Words oft creep in one dull Line, / While they ring round the same unvary'd Chimes" - that is, how the sound of the poem ("Chimes") is "unvary'd" from other works of literature, and uses the "expected Rhymes". One particular example he gives are the "Expletives" (which are exclamations, especially 'cuss' words) who give their "feeble Aid" to both the tiring "oft the Ear" and the sound of the poem that Pope is so focused on here.