Many report templates include a Content tab with Detail level options. This feature is aimed at focusing the report at different levels.
example
A report template on mailbox content such as General Mailbox Content includes three detail levels: Mailbox, Folder, Item. If you select Mailbox, it will only display the mailbox name and a total item count and size, whereas it would display a list of items within the mailboxes you selected if you had selected Item.
The same principle applies to all other templates that have a Detail level option: the more you go down the list, the more detailed the report will be.