OK, before you think, "snoresville" and head for the "Steaminess Rating," let us explain. A huge part of the effect of this poem is due to the rhythm of the words and the rhyme, and "quatrains in irregular iambic tetrameter" is just the fancy way of describing that rhythm.
A quatrain is a four-line stanza. This poem is broken into four stanzas, each with four lines. It's awfully symmetrical. Each quatrain can be broken down further according to its rhyme scheme: the rhyme is a regular ABCB – the second and fourth lines always rhyme. Check out this example from the first stanza:
Break, break, break, (A)
On thy cold gray stones, O Sea! (B)
And I would that my tongue could utter (C)
The thoughts that arise in me. (B)
See how the end words for lines 2 and 4 rhyme? In every stanza, the second and fourth lines rhyme.
But that's where the symmetrical regularity ends. The meter of the poem – the pattern of stressed and unstressed syllables – is all over the place. Almost every line has three stressed syllables, which is why we call it "trimeter" (tri = three). If we bold the syllables that you'd naturally emphasize when reading it out loud, you'll see that there are three stresses. Check it out:
And the state-ly ships go on
So what's so irregular about it, you ask? Well, even though there are usually three stressed syllables per line, the total number of syllables per line varies quite a lot. The first line, for example, has only three syllables, total, and they're all stressed:
Break, break, break.
Compare that to line 14:
At the foot of thy crags, O Sea!
Why all the irregularity, then? Believe us, it's not because Tennyson didn't know any better. Quite the opposite, in fact. The regular stresses, but irregular total number of syllables, could be seen as a throwback to Anglo-Saxon poetry, like Beowulf or "The Wanderer." Could Tennyson have consciously been mimicking the Old English form in "Break, Break, Break"? Why would he do this? What would the ancient form suggest?
The rhythm of the lines often reflects the mood of the poem. The abrupt first line, "Break, break, break" sounds "broken" up both by the commas between the words, and by the absence of other unstressed syllables. Line 14, on the other hand, with its lilting rhythm, suggests the rolling of the waves at the shore before they break on the rocks. The irregularity and slight unpredictability of the rhythm could highlight the unpredictability of the sea, or the instability of the speaker's state of mind. Or maybe both. What do you think?