There are things you can say and things you cannot say, not because there are speech controls (though there are) but because holding certain views makes you anathema and dismissable. This leads to less influence and effectiveness. 

The Overton window is a way of mapping sayable opinions. The goal of advocacy is to stay within the window while moving it just ever so much. For example, if you are writing about monetary policy, you should say that the Fed should not immediately reduce rates for fear of igniting inflation. You can really think that the Fed should be abolished but saying that is inconsistent with the demands of polite society. 

That's only one example of a million. 

To notice and comply with the Overton window is not the same as merely favoring incremental change over dramatic reform. There is not and should never be an issue with marginal change. That's not what is at stake. 

To be aware of the Overton window, and fit within it, means to curate your own advocacy. You should do so in a way that is designed to comply with a structure of opinion that is pre-existing as a kind of template we are all given. It means to craft a strategy specifically designed to game the system, which is said to operate according to acceptable and unacceptable opinionizing. 

In every area of social, economic, and political life, we find a form of compliance with strategic considerations seemingly dictated by this Window. There is no sense in spouting off opinions that offend or trigger people because they will just dismiss you as not credible. But if you keep your eye on the Window – as if you can know it, see it, manage it – you might succeed in expanding it a bit here and there and thereby achieve your goals eventually.