What does "run on line," in poem, mean?
I'm no expert on poetry, but I would guess that a run on line is a sentence that wraps around to the next line, rather than coming to a convenient stop at the end of the line.
Another word for it is enjambment and it's exactly as Uriel described. Eliot's "The Waste Land" begins with the lines enjambed:
APRIL is the cruellest month, breeding
Lilacs out of the dead land, mixing
Memory and desire, stirring
Dull roots with spring rain.