This graph shows the daily ratio of [ page views ] per [ visitor session ] for your site over a time period of your choice (you can change that period by selecting a different date range from the top of the page). Each bar represents one day, and the bar's height represents the ratio on your site for that day (the exact number can be read at the bottom of each bar or by hovering your mouse over the bar)
Note that Saturdays and Sundays are represented by slightly lighter colored bars
If the period you are looking at is greater than 60 days, a red line will appear superimposed to the graph. It represents the 28-day rolling average of your daily traffic. Since it smoothes out the daily variations it gives you a better idea of your long-term traffic trends.
A high ratio is a sign that your site captures your visitors interest since a visitor who spends more time on your site and goes deeper inside will generate more page views than an visitor who just glanced at the home page and left. In that respect, this report is the opposite of the bounce rate report.
Note that there is no 'right' value for the page views per visitor ratio. It's used to compare traffic patterns over time on a given site. For instance, if you redesign your navigation and the page views per visitor ratio suddenly drops, then it is a sign that your visitors may be confused and unable to navigate the new site. Seeing that report drop is an alarm bell telling you that something is wrong.