An on-screen whiteboard is a rectangular white space provided by the tutoring software/application on the screen of the computer, laptop or any other electronic gadgets where the tutors can write or edit using tools like virtual pen/pencil/ eraser etc.
PowerPoint presentations, videos and other forms of media can also be played on the whiteboard to make the session more interesting.
What is most effective is that the student on the other side can also do the same which makes it a highly interactive tutoring session.