Vertical lines have undefined (or no) slope always, so this is a true statement. To see why, look at a horizontal line (which is perpendicular). They have zero slope. That comes from the y = mx + b slope intercept form and their equation being y = b, and b is some number.
This relationship is always true: a vertical line will have no slope, and "the slope is undefined" or "the line has no slope" means that the line is vertical. (By the way, all vertical lines are of the form "x = some number", and "x = some number" means the line is vertical.
