Help Information:

Sessions Graph
This report shows the trend of recent activity on your website in terms of Visitor Sessions over time. The default timeframe is one week, but this can easily be changed in the Date Range control area.

Calculation Methodology
A 'Session' is defined as a series of clicks on your site by an individual visitor during a specific period of time. A Session is initiated when the visitor arrives at your site, and it ends when the browser is closed or there is a period of inactivity. Sessions quantities will vary to some degree based on what type of visitor tracking method is employed. For the most accurate numbers, the Urchin Traffic Monitor (UTM) should be used.

 Urchin Documentation Center (help.urchin.com)    Glossary of Terms