Complex sentence: The Overton window is a political theory that describes as a narrow "window" the range of ideas that the public will find acceptable, and that states that the political viability of an idea is defined primarily by this rather than by politicians' individual preferences.
Split into simpler sentences: The Overton window is a political theory that describes as a narrow "window" the range of ideas that the public will accept. It also says that the political viability of an idea is due mainly to this window rather than to politicians' individual preferences.