An editor is a window within Epic's GUI dedicated to a database record, and remains open only as long as you are viewing or editing the record. Some editors (e.g. the project editor) show other records relating to the parent record.
Editors normally opens in a read-only mode. If you wish to edit the record you open the read-write equivalent editor. This is an example of progressive disclosure.
Whilst you have a read-write editor open, no-one else can open another read-write editor on the same record.