The red line (called Magic Line by CKEditor) appears before or after certain elements in Edit Mode to makes it easier to place your cursor and add more content to the page.
The Expert editor contains a red line feature that allows users to add paragraph breaks above or below formatted sections on a page.
The red line tool appears above and below formatting blocks (other than comment and custom blocks). Trigger the red line by hovering above or below a formatting block. The red line tool allows users to add space before and after formatting blocks, tables and table cells.