Shakespeare wrote his sonnets (and much of the dialogue in his plays) in a meter called iambic pentameter. Each line of poetry contains ten syllables comprising five iambs. An iamb is a group of two syllables in which the second syllable is stressed. Sometimes it's helpful to think of the rhythm as a heartbeat: buhDUM buhDUM buhDUM buhDUM buhDUM.

Sonnet 130, which begins "My mistress' eyes are nothing like the sun," maintains this meter almost entirely throughout. Try reading that line aloud, for instance. Even without trying, English speakers will naturally stress the correct syllables: "my MIStress EYES are NOthing LIKE the SUN."

The rhythm breaks, however, in the second to last line, or the first line of the final couplet.

"And yet, by heaven, I think my love as rare

As any she belied with false compare."

There are eleven syllables in that bold-typed line. The single syllable words all fit neatly into iambs, but the "-en" at the end of "heaven" is metrically out of place. Though poets, Shakespeare included, do sometimes treat the word "heaven" as a one syllable word, when this is the case it is usually written as "heav'n." The extra unstressed syllable here can only be intentional.

Generally, when Shakespeare chooses to break meter, it is used to signal great emotional distress or a significant change in the speaker. In this case, the first three quatrains, and therefore the bulk of the poem, are spent unfavorably comparing the speaker's lover to various poetic and romantic cliches. Her blush isn't like roses, her voice isn't like music, her breath smells nothing like perfume. It is all rather insulting and extremely humorous. Only in the final lines do the speaker's true intentions in writing this unflattering litany become clear: he refuses to idolize her and compare her to a thousand things that may be pretty but wouldn't be accurate. Instead, he sees her for who she really is, no metaphors needed, and his love for her is stronger for it.

This ending is a sweet twist that a first time reader may not see coming, and the change in meter is a formal reflection of the change in tone and content. It is also notable that the words that break the meter, "by heaven," are an oath or swear. That the meter breaks on these words could be interpreted as the speaker becoming too overcome with emotion to maintain a measured line of verse.

The metrical pattern of Shakespeare's Sonnet 130 is actually perfect iambic pentameter all the way through. An iamb is two syllables together, one with soft and one with hard accents. Iambic pentameter is a line of five iambs. Sonnets are traditionally written in iambic pentameter, and Sonnet 130 does not break this pattern, as we see in the second stanza:

I have' / seen ro' / -ses dam' / -ask'd , red' / and white',
But no' / such ro' / -ses see' / I in' / her cheeks';
And in' / some per' / -fumes is' / there more' / de -light'
Than in' / the breath' / that from' / my mis' / -tress reeks'.

However, that being said, there are a couple of unusual choices in the meter that may throw the reader off. For instance, in American English the word "coral" is typically stressed on the first syllable: cor' al. However, in line two, Shakespeare stresses the second syllable of coral:

Co -ral' / is far' / more red' / than her' / lips red'

We must remember that some pronunciations for Renaissance Early Modern English are unknown to us. In that time period, the second syllable may have been drawn out farther than it is today due to the spoken accent.

Another spot that looks unusual is where he accents the preposition "on" in line 12:

My mis' / -tress, when' / she walks',/ treads on' / the ground'

This is unusual because normally in English, we would emphasize either a noun,  a verb, or an adjective, rather than a preposition. However, being that both "tread" and "on" are monosyllabic words, even this pair is a perfect iamb, which maintains his consistent use of iambic pentameter.