A summary field aggregates the instances of another field (e.g. a database field or formula field) by calling a summary function.
The "Summary Field Properties" dialog helps you to create a summary/running total field.

To create a new summary field, select the node "Summary Fields" in the field browser tree and click on the new button of the fields browser.
Each summary field has to have a unique name. Enter the name in the "Summary Name" field, for example "Sum of <field name>", the "Field to Summarize" and the "Summary Function" itself.

To edit an already existing summary field, click on the summary field in the fields browser and then on the edit button.

In the upper part of the dialog you can choose the (database or formula) field on which the summary function should operate as well as the summary function itself.

If you do not want to call the summary function for each record, then aggregate it by using Group by (below you can see the list of valid conditions).

The default mode is to summarize the data of all rows or by a group. To group the summary field you need to add a group name field into the "Grouping by" field.

Another option is to choose the "Running Total" option on a summary field. By clicking on the "Running Total" check box, the summary field will be forced to be evaluated for every row of the report.
If you want a running total field, please check the "Running Total" check box and enter the condition on which the summary value should be reset. Below you can see the list of valid conditions.

Running total summary fields can have conditions when to evaluate it's value. Below you can see valid conditions for "Evaluate on change of".

Running total summary fields can have conditions when to reset it's value. Below you can see valid conditions for "Reset on change of".