When user first enters app studio of a site, they need to click the launchpad and navigate to the app they wish to make changes on. Assuming the app has fields in it already user can go straight to reports, user can then change report to a List Report.
In a List report, if the summary expression is defined (e.g. Sum) then the list report will show a subtotal at the grouping level. Summary Expression can only be found in App studio and has four functions (Average, Sum, Minimum and Maximum).In order for summary expressions to be used field needs to be number fields.
When a grouping filter has been applied - there will also be a breakdown of the summary expressions per grouping. See image below: