AutoFill by Priority Group means that the employees you designate as Group 1 will be assigned to their preferred schedules first (up to their maximum hours per week with one shift per day), and after that is completed, the Group 2 employees will be assigned to the remaining shifts, then Group 3, etc.

If after all Groups have been considered, there are still shifts remaining and employees available, AutoFill will run the entire process again to assign the rest of your shifts.  This is all accomplished in a matter of seconds.

With Priority Group scheduling, your employees in higher priority groups can be given preferential schedule assignments.  This will also help your full-time employees to get their maximum hours per week.

To AutoFill based on Priority Group:
(Be sure you have edited your Employees and set their AutoFill priority groups)
From any weekly Unpublished Schedule View: 

1. Click AutoFill
2. Click the radio button next to "By Priority Group" 
3. Click the AutoFill button