An editor-at-large is a journalist who contributes content to a publication. Sometimes such an editor is called a roving reporter or roving editor.
Unlike an editor who works on a publication from day to day and is hands-on, an editor-at-large contributes content also on a semi-regular basis and has less of a say in matters such as layout, pictures or the publication's direction.
Editors-at-large are more independent; they are allowed their own preferences in the content they have to generate, and they do not always have to pitch their ideas to the main editor. Though they are still subject to the direction and oversight of chief editors and executive editors, they frequently come up with ideas for other writers to research and write. "At large" means the editor has no specific assignments, but rather works on whatever interests them.