The Report Detail tab will rename to whichever report you've selected, in my example above it will change to London Trip and remain active until you choose another Report, or refresh your browser.
This is where users with Expense: Submitter permission will see the Expenses within a Report they have uploaded/created. As a user with Expense: Export, Schedule & Reimburse permission, this is where you will be able to see the details of the individual Expenses within a Report.