Document or page sharing is a feature unique to Riff. It allows participants to share most Web-addressable resources on screen such that everyone in the meeting can use that resource.
For example, if you use the document sharing feature to load an open access or permissioned Google doc, then all participants can have access to that resource in the window and can work on that document collaboratively.
Another example: in online courses, the document sharing window is used for delivering simulations, games, whiteboards, and other resources that groups need to work on together.
The screen sharing button is the middle button under your own video display.