_NET_SHOWING_DESKTOP
The root window property saying whether the window manager is currently hiding every window to reveal the desktop: the mode behind the Show Desktop button.
The button came from Windows: the Quick Launch bar with Show Desktop arrived with Internet Explorer 4's desktop update in 1997 and Win+D with Windows 98, and the panels in GNOME and KDE wanted the same thing. But a panel cannot minimise other people's windows on its own without treading on the window manager's toes, and above all it cannot undo it gracefully. EWMH therefore made the mode a state of the window manager: a CARDINAL on the root window, 1 or 0, which a panel or pager requests with a ClientMessage. The manager chooses how to hide the windows, by minimising, unmapping or lowering them beneath the desktop window, and must remember which ones so that only those come back when the mode is switched off.
Metacity minimised and remembered; KWin has had two variants since Plasma 5, one that minimises and one that merely sweeps the windows aside for a moment. Most leave the mode the instant the user activates a window, which is why clicking a desktop icon that opens a program also brings everything else back, to widespread annoyance. Openbox has ToggleShowDesktop, Xfce Ctrl+Alt+D, and wmctrl -k on requests the mode from a shell. GNOME 3 removed both the desktop icons and the button in 2011 and left it to extensions to put them back; Mutter still honours the property.