How to interpret blue lines that show motion in Program Manager

I have always been puzzled by the blue lines (straight or curved) that appear when I'm editting video.  I cannot find documentation on what information these lines show me and what they are guiding me to do.  I'm do a lot of zooming and focusing on particular moments in a long video (in this case an hour and a quarter long play).  What makes them appear? Why do they seem cumulative in a give editing session? What about them tells me what I need to consider in order to make cleaner and smoother (or sharper and more abrupt in cases where that is appropriate..) This is likely to be too complicated at topic to consider here, but I would appreciate direction toward a good explanation of the purpose and use of what are sometimes called "motion guides,"