Basically something that us grey beards like in several window managers, it is not supported in GNOME since the version 3.0 reboot, and relates to minimizing a window to the title bar.
You can move the title bar around, and depending on the window manager either double clik to drop down again its contents, or by leaving the mouse pointer for a few seconds hover it, it will temporarly reveal its contents.
https://en.wikipedia.org/wiki/WindowShade
