_NET_WM_ICON_GEOMETRY
The property where a taskbar records where a window's button sits, so the window manager knows where to shrink the window to when minimising it.
Under twm the icon lived on the root window and the window manager knew where, because it had put it there. Once taskbars became separate programs that knowledge vanished: the window manager does the minimising, but the button the window is supposed to fly into is drawn by an entirely different process. EWMH solved it with four numbers — x, y, width and height — which the taskbar writes on each client window it shows a button for, and updates whenever the buttons move.
The window manager reads the value on iconify and animates towards that rectangle, with an outline in the old style or a full shrinking image under a compositing manager. Without the property the animation falls back on something arbitrary, often the bottom of the screen, and the window vanishes into a spot with nothing to click on.