What is a line? In screenwriting, IT IS something someone says. In poetry, too; but a line is also a unit of rhythm, represented on a page by a return to the left margin, somewhat the way sheet music represents measures with vertical lines.
Maybe it's easier to show examples of the poetic line than to define it. As with measures in music, all lines are not alike. Some are slow and some are fast, and in some the sound of the sentence stops at the end, and in some it blithely keeps going right over the line, just as this little poem by Robert Frost says:
In a Poem
The sentencing goes blithely on its way,
And takes the playfully objected rhyme
As surely as it takes the stroke and time
In having its undeviable say.
Frost is partly showing off by giving a brilliant example of how what he describes should be done. All four of his lines follow the same pattern, and no two of them follow it in the same way. Like a jazz solo, the poem combines striking variations in movement with a rhythm that is "undeviable." The poem offers good advice about how to hear poetry, and it also enjoys its own virtuosity.
Similarly, the much earlier master Ben Jonson (1572-1637) shows what he can do, flamboyantly -- like a dancer leaping -- in order to chide a woman who has disrespected his picture. Poor Jonson sounded better than he looked. Cupid, the god of love, is supposed to be blind -- but in this case:
My Picture Left in Scotland
I now think love is rather deaf, than blind,
For else it could not be,
Whom I adore so much, should so slight me,