The whiteboard feature will allow you to share a whiteboard that you and other participants (if allowed) can annotate on.
If you have simultaneous screen sharing on, multiple participants will be able to share a whiteboard at once. However, to see two whiteboards at once, or your own whiteboard and another participant's, you will need to have dual monitors enabled.
Note: Whiteboard is currently available for iPad only. If you are on an iPhone, you will not see the Whiteboard option.