Every time you run a program, it opens a window on the desktop and it adds a button to the task bar. Press the task bar button to hide and show the program window.
Open a program like Notepad.
Press the task bar button to hide the Notepad window.
Press the task bar button again to show the Notepad window again.
When you open another program like Paint, it may open on top of Notepad.
Press the Notepad task bar button to show the Notepad window on top of everything else.
This completes showing on top.