An editor is a mediator. She stands between the writer and the reader and helps them to understand each other.
If she works for a publishing house she also mediates between the publisher, other staff, and the writer.What traits make a good editor?
The qualities above are important for all editors. If an editor works for a self-publishing writer, he works unsupervised. He must know his limitations and be sure he is working within the bounds of his skills. When you choose an editor, make sure you choose someone with the skill set and personality you need for your project.
There are many levels of editing and each editor will have stronger or weaker skills in each of those levels. You can learn more about the various levels of editing on the FAQs page.