This is the main working area of the editor, where your UI wireframe comes to life. Once you add UI controls to it, you can move them, resize them and tweak them until your UI wireframe is ready. See Adding and Arranging UI Controls to learn more.
The canvas grows and shrinks with your application window, so you can make room for bigger wireframes if you need to. The maximum size of the canvas is 10000x10000 pixels.
Note: If you are working on a small screen or have very large wireframes you might want to read this section on hiding the panels to make more room for the canvas.