The summary reports are quite similar to the ones you can include on your Dashboard, with the main difference that they display more data and can be customized to include more or less data by means of using the Date Range tool (just below the Summary Overview tab).

When you click on a Summary report, for instance Visitors per day, at the right top corner next to its name you will find the Help button that opens a window with tips and explanation about the usage of the particular report and an explanation of the terms used in it.