Some lines of poetry end at a natural point of pause, accompanied by punctuation, such as a period, comma, or semi-colon. These are called “end-stopped" lines. The first and last lines in this first stanza from Mark Doty's poem “The Embrace" are end-stopped:
You weren't well or really ill yet either;
just a little tired, your handsomeness
tinged by grief or anticipation, which brought
to your face a thoughtful, deepening grace.
The other two lines in this stanza—the second and third—are “enjambed." That means one line run right into the next. The line break doesn't occur at a natural pause. Instead, it separates pairs of words that, in prose, would read without a pause.
Poets use these different kinds of lines—end-stopped and enjambed—to create specific effects. In the first stanza of Doty's “The Embrace" the two enjambed lines create a forward motion. Some poets will use several enjambed lines in a row to create even more urgency. Once you get to the end of the sentence and to the end-stopped line, there's a brief release in tension. You are invited to pause for a moment and consider.
Be thoughtful in where you break the line. Your choices impact the reader's experience of the flow and motion of the poem.