The phrase "default headline" refers to a pre-set or automatic title that appears when no specific title is provided. It's like a placeholder headline that is used as a default option when something else is not specified. Full definition
If you have used default headline (name of the company and your current position), it is time for actions! (skillroads.com)
The default headline is your current position as a student or professional. (topresume.com)
They haven't changed the default headline automatically populated for this spot, based upon their current, or most recent job title. (executivecareerbrand.com)