This graph shows the visits(*) on your site over a time period of your choice. You can change that period by selecting a different date range from the calendar icon at the top of the page.
* A 'visit' or 'session' is started when a visitor opens your site in a browser. It is terminated when that same visitor (defined by his IP address or a cookie) has been inactive for more than 30 minutes. If the visitor comes back to your site after more than 30 minutes, a new session is counted.
The exact number of visits can be read at the bottom of each bar or by hovering your mouse over the bar.
In the case of daily reports, if the period you are looking at is greater than 60 days, a red line will appear superimposed on the graph. It represents the 28-day rolling average of your daily traffic. Since it smooths out the daily variations it gives you a better idea of your long-term traffic trends.