Your headline is meant to be the space where you can introduce what you do professionally. When clients are doing a freelancer search, they'll see it displayed just below your name as a sort of personal subtitle or tagline.
The simplest way to introduce yourself would be to just name your basic job title, like “Ghostwriter” or “Corporate Tax Accountant.” If you have many freelancing skills, you could choose to list those here, like so: “Photoshop | Adobe | FinalCut Pro.” If you want to be eye-catching, you could share a slogan (as long as it makes it clear what you do), like “Editing You Can Count On.”