A window into the world of ideas that frames what people are prepared to entertain and consider possible. Ideas outside the window are not seriously considered.
“Really revolutionary ideas simply roll off men’s minds like water off a duck.”
Thus, the act of ‘shifting’ the Overton window involves the work from people and ideas outside this window to shift what if considered acceptable by the general public. This is mostly talked about in the realm of policy but applies to other fields as well.