The editor pane is where you edit your documents opened or created by the Oxygen XML Author plugin Eclipse plugin. You know the document is associated with Oxygen XML Author plugin from the special icon displayed in the editor's title bar which has the same graphic pattern painted with different colors for different types of documents.
This pane has three different modes of displaying and editing the content of a document available as different tabs at the bottom left margin of the editor panel: Text mode, Grid Mode, Author mode (CSS based tagless editor).