Maximizes the window. If the window was already maximized, then this function does nothing.
On X11, asks the window manager to maximize window, if the window manager supports this operation. Not all window managers support this, and some deliberately ignore it or don’t have a concept of “maximized”; so you can’t rely on the maximization actually happening. But it will happen with most standard window managers, and GDK makes a best effort to get it to happen.
On Windows, reliably maximizes the window.