System dividers (also known as system separators) are thick diagonal lines that appear in large ensemble scores (usually orchestra, opera, etc.). See attached for a sample screenshot.
They are used for scores that alternate between pages with a single system per page (when the full orchestra is playing) to pages with multiple systems per page (because only a few instruments are playing and the rest of the instrument staves are hidden). This helps the conductor follow the score.
They are not typically used in small ensemble scores where the number of staves per page does not vary.