The canvas is the main workspace, the background onto which all other elements will be placed. You can think of it as a blank sheet of paper.

The canvas itself is nothing more than a normal image frame, locked to the dimensions of the canvas size.

Like any other image frame, it can be tinted a solid color, be given attributes from the Image editor, or accept an image or a texture.

 
If you want to use an image already dropped onto the canvas, right-mouse select the image and "Set as Canvas Background," or pick  the image up by the frame number and drop it outside the canvas. The image (or texture) will snap to fill the entire frame.

To move the image to the canvas as a normal frame, right-mouse select an empty space on the background image, and select "Set as Regular Frame".

To reset the canvas back to the default, right-mouse select outside the canvas area, and reset the canvas to white.
 

This section of the user's manual will teach you how to change the size and appearance of your canvas: