Icons represent windows that are temporarily hidden. The green with black dots covering most of the screen is called the background . Move the mouse to any area of the background and try clicking the three mouse buttons each at a different time. Each button brings up a different menu of applications. Do not try any of these now , you can try them on your own later.
Most windows have title bars across the top of the window.
Move the mouse to the title bar for the calculator window. Click and hold the left mouse button on the word Calculator. Move the mouse and the window should follow. Let go and the window will be in its new position. Another useful window command is resizing the window.