An online editor is responsible for adding the finishing elements of a video sequence at full quality. This includes title elements, color correction and visual effects.
Typically an online editor does not make creative changes to the shot choice in an edit, but is concerned with the overall polish of the final piece.
Depending on the production pipeline an online editor may simply be responsible for conforming all final video, or they may actually perform the color correction, titling and effects work.
An online editor is also responsible for ensuring that a project conforms to legal broadcast standards before delivery and distribution.