The help definition actually isn't too bad...so I am going with it.
P6 enables you to define your organization's global financial periods in the Financial Periods dialog box. Customized financial periods provide more accurate display and reporting of actual units and costs. Rather than spreading costs evenly throughout the length of a project, users can view how actual costs were incurred by customized financial period.
If your organization always updates the schedule according to the same time interval (i.e., every week, every month, every quarter, etc.), you can quickly create a batch of financial periods. If the schedule may be updated irregularly, you can create a single financial period at any time.
When financial periods exist in the Financial Period dictionary, users can store period performance for any defined period. When you store period performance, actual units and costs are stored as past period actuals. The past period actual values can be edited for any financial period in columns of the Activity Table and Activity Details, Resources tab. Past period actual spreads per financial period can be viewed in the Activity Usage Spreadsheet and Resource Usage Spreadsheet, and past period actual data can be displayed in profiles, time-distributed reports, and the Tracking view.