When you establish an expected duration for a workflow, the system will use that to automatically calculate the due date for the checklist runs.
Example without Business Days:
If you have an expected duration of 5 days for a workflow that is started on a Thursday, the system will calculate the due date to be Tuesday of the following week as it will include Saturday and Sunday.
Example with Business Days:
Again we have a workflow with an expected duration of 5 days. By using the Business Days setting, you can exclude Saturday and Sunday in this calculation - resulting in a due date of the following Thursday.
Business Days can be configured specifically for each of your workflows in the workflow settings.