Hoping to save some fellow devs a bit of confusion here. When I hit this lesson, I had almost all of my Gizmos turned on (I think this is the default).
I spent a while wondering why adding my canvas caused a vertical and horizontal line to shoot out of 0,0,0 even in my game view, and eventually caught the Canvas gizmo as the issue.
I suspect this is the kind of thing that most people configure once in Unity and forget is there, thus, not explicitly mentioned. Hope that helps the next person! 
By default it’s enabled in Scene view and disabled in Game view.
The canvas does have the lower left corner in world 0,0 so yup that’s the vertical and horizontal line you see there.
If you also see lines on top/bottom/sides then that could also be the Camera frustum gizmo.
I think I know why you had them turned on - in one of the first lectures, when we used Debug.DrawLine to show visual for the grid, CodeMonkey mentioned to turn on the Gizmos to have them visible. They are normally not visible in Game mode by default.