All statistics reports work in the same way.
As enrolment reports are the most commonly used, these are explained in details in the Enrolment Statistics Reports help notes.

To delete a selection, highlight the value required and click the "Remove" button.
To include a selection, click on the tab for the category required and click the "Add" button.
A selection form will allow you to choose the value required, eg. for territories.

Sorting Options: 

These will determine the sequence of records on the report.

Examples of Reports: