Iambic Pentameter
Heroic couplets are the ideal form to put a long, narrative poem in. We mention this in our "Form and Meter" section, too. The iambic pentameter marches us steadily forward at a pace that's not too slow (add one more foot, and it might be. Try reading Henry Wadsworth Longfellow's Evangeline, written in iambic hexameter; it will feel much longer than it is), and not too fast (think about galloping lines like Alfred, Lord Tennyson's dactylic dimeter in "The Charge of the Light Brigade"). Heroic couplets give you stamina; and with a poem that's over 20,000 lines long like this one, you will definitely need it.
Couplets: Rhymes, End-Stopping, Enjambment
While those regular, five iambs per line might help to move us forward, the heroic couplet has one characteristic that might work against us getting through the poem in a steady fashion: its rhymes.
Think about it: if each couplet is composed of two rhyming lines, then those lines are going to feel like a complete unit in and of themselves. This quality of heroic couplets—or of any set of lines that you feel might be able to stand alone and apart from the poem on their own—is called epigrammatic (i.e., the lines would make a catchy epigram for something else). That's great, but it only gets you so far in a 20,000 line poem.
Pope counters the epigrammatic feel of the couplets by strategically using two poetic techniques at the end of his lines: end-stopping (where he uses a grammatical pause, like a colon, semi-colon, comma, a dash, or a period, at the end of a line), or enjambment (where there's no punctuation at the end of the line, so you're not supposed to pause as you're reading it).
Here's a great example of how this works, at the very end of the poem, when the speaker brags to Belinda about how much better off her hair is now that it's the subject of a poem:
Then cease, bright Nymph! To mourn thy ravish'd Hair
Which adds new Glory to the shining Sphere!
Not all the Tresses that fair Head can boast
Shall draw such envy as the Lock you lost. (V.141–144)
The end of the first line is enjambed; no punctuation after "Hair", so when you read it, you have to wrap right to the beginning of the next line without stopping. That's fine, because Pope wants you to build some momentum before you hit the emphatic end-stopping of the next line, "the shining Sphere."
Feel the contrast in effect? Within the strict overall rhythm, Pope is varying the ends of his lines, making his reader speed up or slow down in subtle ways that "break" the form of the couplet without really breaking it.