An independent editor you hire directly to improve your manuscript, rather than one assigned by a publishing house.
A freelance editor is a self-employed professional who provides editing services directly to authors. Unlike in-house editors at publishing companies, freelancers work on a contract basis and you choose (and pay) them yourself. They come in several flavors depending on what level of editing your manuscript needs, from big-picture story restructuring to final proofreading. For self-published authors, a freelance editor is essential. For traditionally published authors, hiring one before querying agents can make the difference between a form rejection and an offer.
You cannot effectively edit your own work. You're too close to it, and your brain will fill in gaps and smooth over problems that a reader would stumble on. A freelance editor brings trained, objective eyes to your manuscript and catches issues you literally cannot see. Skipping professional editing is the single most common mistake self-published authors make, and readers notice immediately.
If your story has structural problems, fixing typos won't help. Start with the deepest level of editing your manuscript needs and work outward. Structure first, then prose, then correctness, then typos.
Ask for a sample edit (most editors will do 1,000-2,000 words for free or a small fee). Judge the quality of their feedback, not just their rate. A cheap editor who misses major issues costs you more in the long run.
Self-edit as thoroughly as you can before hiring anyone. Fix the problems you already know about. Beta reader feedback is also worth getting first. Don't pay a professional to catch things you could have caught yourself.
Take one chapter from your work-in-progress and read it four separate times, each time looking for a different level of editing: first for story and structure issues, then for sentence-level clarity and flow, then for grammar and consistency, then for typos. Notice how different each pass feels. This is why professional editors specialize.