The Operator users allow you to add an administrator user to the system that is hidden from the system, and as such do not act as an active license in the system. 

Employees set to this mode will be able to access the timesheet, rosters, and payroll modules however not appear in any of these as a visible employee.

To set this option for an employee, open their employee's profile on the Personal tab as shown below.

Please note that this setting can only be modified if the employee has never been included in the list where a payroll has been processed, this includes payrolls where their was no payment made to the employee in question as a record of the employee being included is still saved in the background.  

If you have employees that have been previously set to a standard employee and have been processed along with your other employees, and you want to adjust this going forward you will need to re-create/duplicate the relevant employees, set the operator setting and then terminate the previous instances of the employee in the system 
Important note: You cannot terminate the currently logged in user, you will need to set up the replacement one and log in to it before completing the process).

You can check your active users in the system at any time by Running the Licence Count Report.