This means that your Operate is set to charge old Accounts for a number of months after termination. This is useful when you need to charge old clients for any service rendered after move-out/account termination, such as correnspondence forwarding. 

If charges were added before the account termination (prior to the end date that has been added to the Account record - which represents the date of Account termination), the customer should still be billed for them, since they are part of their normal billing, for a period within which they were still a customer.. It will not be possible to charge  the account after the end date has passed (for example licences, bookings, charge sheets, invoices, etc.), including through the Bill Run. 

To check this setting, please go to Operate's Settings > General > System > Operate Settings > Billing Settings. 

Under the Accounts page section, check for how long you allow old clients to be billed. Set this to 0 if you do not want to use the option.   

If you enable this feature by entering a number greater than "0" in the field, then Operate will keep charging terminated accounts for the amount of months you configure here. 

Terminated accounts refers to accounts which have an end date set to the Account record.  Learn more about terminating Accounts in Operate