A Ttk widget that goes in the south-east corner of a window and allows you to resize the toplevel that contains it. Note that on OSX, all windows always have something like this by default.
http://www.tcl.tk/man/tcl8.5/TkCmd/ttk_sizegrip.htm documents the widget, which is used in a megawidget or application window to provide a resizing capability.
Formerly called a Tile widget.