On the filter bar to the left side of the main screen, you can select a date filter to show billed items for a specific date range rather than showing billing items from all time. Once you apply a date filter to create a client report using a 'From and Through Dates', any un-archived transactions before the 'From Date' will be totaled up and displayed as a Previous Balance for the first line item on the report.
Look at this screen shot of the main screen and you'll see a date filter has been selected on the left. Items that are older than "this month" have been totaled and are summarized in the Previous Balance line at the top of the report shown.
One use of Previous Balance is to charge a penalty for Overdue Interest. Click here for the details on using Overdue Interest in Legal Billing.
This help article applies to Legal Billing 7 or newer