It's a Table of Contents.
No really... as you scroll through the menu's with the buttons on your dash, the highlighted bar (off color - orange in my car) will move up and down that "vertical bar chart" to let you know what page you are on. I believe there are 7 or 8 total.
I didn't know this either, until I looked it up in the Owners Manual. Then I was like, "Duh."