The board
This is the main working area of the editor. You can add, move, resize, and tweak any UI element added to the board. Check out Adding and Arranging UI Elements to learn more.
The board is large - it's 20000px x 20000px - and flexible so you can scroll in every direction to make room for bigger wireframes or more UI elements if you need to.
Note: If you are working on a small screen or have very large wireframes you might want to check out this section on hiding the panels to make more room for the board.