You can find a list of the functions of each button in the extension here
The basic concept of the extension is to be able to prepare your canvas with any items that you want to be able to present, these can be any number of files and / or the webcam.
When the extension loads you will be presented with a dialog where you can search your drives for files that you can add to the canvas:
Search you drive for files that you want to add the the canvas
Choose the file that you want to add to the canvas
Drag the file to where you would like on the canvas using the top bar and resize from the bottom right
This icon will toggle on and off the camera on the canvas
Watch this quick video to see how: