Clicking the right button of the mouse opens up what is called a 'context menu.' A sample image is shown below. If you accidentally open a context menu, you can close it again by moving the mouse cursor off of the black menu box and then clicking the left mouse button.
Try right-clicking now to open a context menu, then close it.
The context menu often allows you to perform common operations that are also available from the menu bar at the top of the screen.
This context menu is from the Firefox web browser. The options available via right click will be different for each app.
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