In poetry, a line break is the end of a line in a poem. The poet chooses to break each line in a specific location for a specific purpose (or multiple purposes).
There are two main types of line breaks: end-stopped and enjambed. An end-stopped line generally ends where a phrase or sentence in the poem ends. They tend to be marked by periods (the true end-stopped line, being a full stop) or other punctuation marks. This technique provides a good deal of emphasis, and often makes the sentences used seem factual. It can also make a poem seem harsh and choppy, with a heavy, serious rhythm.
Take, for instance, the first lines of John Berryman's Life, friends, is boring. We must not say so.
Life, friends, is boring. We must not say so.
After all, the sky flashes, the great sea yearns,
we ourselves flash and yearn,
Berryman seems to take the first line as the premise or even thesis of his poem; the rest of the poem discusses these ideas in detail. The break itself makes the line seem factual; it makes it definite. Berryman can then go on to play with the meaning of such statements in the following lines, which then take the form of evidence, and in the poem as a whole.
An enjambed line, in contrast, lets phrases spill over into new lines wherever they choose. This provides a feeling of movement throughout the poem; it propels the reader forward. It hurries you to the next line; often the first words after an enjambed break are more important than those after which the previous line actually broke. It makes the division between lines more subtle (as opposed to being battering rams yelling LOOK AT ME!, as end-stopped lines can become if you aren't careful). It can obscure a rhyme scheme or even a full sonnet structure.
Back with Berryman, we see that an enjambed line is every bit as useful as an end-stopped:
and moreover my mother told me as a boy
(repeatedly) "Ever to confess you're bored
means you have no
Inner Resources." I conclude now I have no
inner resources, because I am heavy bored.
The line break (and stanza break) preceding "Inner Resources" make these resources a prime focus for the reader's attention. The previous lines were all building toward it: these Inner Resources must be every bit as important as the capital letters in the mother's voice indicate.
Thus end-stopped and enjambed lines may work together to create a series of emphases on key points in the poem. A skillful poet may manipulate them so as to create exactly the impression he or she wants to portray.