After you have powered up your computer and Windows has finished loading (can take 30-90 seconds depending on the computer and software installed), to do something productive, your next step will be to open (start) an application.
You can do this in one of several ways, but the most common of which is to click the "Start" button (typically in the lower-left of the screen), and selecting the application you want to start from the menu that is presented.
Once the application has been started, it will display its main window so that you can interact with it. You interact with it through the keyboard and mouse. (Back in the early days of Windows, to be compliant with the Microsoft Windows GUI standards, all Windows applications were required to always support a keyboard, and the mouse was optional, since not everyone had a mouse.
Once the application's window is displayed, you can do different things with it to suit the way you want to work with it, or "put it aside" without closing it if you need to work on something else for a while (for example, to find a web page from which to copy text or a photo).
You use following buttons on the right side of the title bar to accomplish these window display-state changes:
the Minimize button (puts the window "aside") to clear the desktop for something else,
the Maximize button (makes the window occupy the whole screen when you want the extra screen space),
the Restore button (returns the window from a maximized state to previous "normal" state and location on the screen),
the Close button (closes the application -- save your work before doing this), and finally
the running-application icon in the task bar when the window is minimized (click it to return the window to its state before it was minimized [normal or maximized]).
Note that the Maximize button () changes appearance and becomes the Restore button () when the window is Maximized.
You can freely make the window change states as above without losing any of your work. This is often used simply to clear the desktop for another task, perhaps to copy text from another application, before returning to what you were doing.
the Minimize button (puts the window "aside") to clear the desktop for something else,
the Maximize button (makes the window occupy the whole screen when you want the extra screen space),
the Restore button (returns the window from a maximized state to previous "normal" state and location on the screen),
the Close button (closes the application -- save your work before doing this), and finally
the running-application icon in the task bar when the window is minimized (click it to return the window to its state before it was minimized [normal or maximized]).