due date options in invoicing

Hi, I’ve tried to search for documents on this but I can’t find any explanations.
I’m not sure what the “Due Date Settings” is referring to under Invoicing> Email settings.
Can you please explain what these settings mean?

1) Enable Due Date: Check this to enable due date option for invoices.
2) Default Due Date: Number of days each Invoice is due after the created date. This will automatically set the date in the “Due Date” field. Can be overridden on individual Invoices.

I don’t know what I’m supposed to put here. What do you mean by “due date option?”
Thank you.