Literally, “data about data”; generally taken to mean data which supports or describes other data.
This is a vague term that may have lost some of its meaning over the years. Metadata is often considered to be “extra” or “supporting” data, not the data proper (another source of confusion: there is no opposite term, other than just “data”).
For example, a checkbox for “Show Right Sidebar” on a content object. This is arguably metadata since it’s germaine only to one presentation of the content, not to the content itself.
Editorial metadata is extra data which will never be published, or never even affect the publishing of content, but which supports the editorial process. For instance, a field for “To Do” on a content object which indicates notes of editors on this content will never be displayed, but will be used by editors during their work.