This word is typically used pejoratively to refer to a point of view or set of practices that fall “too far” outside of current mainstream political thinking, and which are therefore considered dangerous (harmful to society), not to mention probably motivated by excessive zeal or some sort of maniacal personality disorder such as a lust for power).
Once a perspective has been labeled extreme, many are disinclined to consider it, with some people unwilling to enter into conversation with anyone who believes it (or they think believes it). Thus any view successfully labeled extreme can thereby find itself left out of the political conversation, in effect silenced.
In another sense, to be extreme just means to be far from the middle (to be near one end) of any distribution that places observed elements in some sort of order. In politics, because many people think of political views as ranging from left to right, “extreme” views typically refers are those views that are “very left” (far left) or “very right” (far right), relative to whatever point of view (however stupid or dangerous) happens at that time to be in the “middle.”
The word “extreme” is also sometimes used to label any view that proposes something very different than what is perceived as normal or within reason. Thus used one way the word “extreme” does not imply either good or bad, or right or wrong.
What if, however, what is thought of as normal at a given time is less wise than many of the views that are far from it? And what if the majority is wrong about something? Some people believe that that does happen, and some of those people are making an effort to encourage citizens to create what they call brave space, meaning space (or spaces) that include “all voices” (all points of view) in conversations about this or that issue. These advocates also call on citizens to be less quick to label as extreme or fanatic those perspectives that makes them uncomfortable.