Bent lines are referring to the lines near the gutter that scan bent if the book isn't flat. Most groups will require scanners to flatten their books, which will minimize the amount of bent lines. If you're given scans that consistently have bent lines near the gutter, ask your group if they'll require you to fix them.
Actually redrawing the line (with a line tool in photoshop) will give you a perfect line -- and the lines in the scan aren't perfect. Since that would stand out, I recommend fixing bent lines by cloning straight lines from the scan. Clone in the lines after rotating and leveling (before or after resizing). An easy example:
For a corner you'll usually also have to extend a vertical line. Also, if there is a texture/gradient, you'll need to clone that in:
And of course, if an image were cut off, you'd need to clone that in as well. That, however, is more advanced cloning, which I'll cover in the typesetting section.
Two Page Spreads ->