Do you mean the vertical ( perpendicular ) line on the grid or do you mean the area defined by the two vertical lines on the grid ?
You are not dumb, it is hard to explain and hard to understand. Therefore i recommended to open the images, enlarge them, read what i have to say about them, and 'see' what i mean. It has to make sense to you for it to work for you.
To answer your question, it is the latter, the area defined by the two vertical (from ball to hole) lines from which your ball will go through when you hit the putt.