At its heart, modern libertarianism is about the morality of force. A libertarian holds that force is justified only to prevent wrongful conduct against persons and their property. In this respect, libertarianism differs drastically from the philosophies of the two predominant American political factions and specifically from the demands for “social justice” on the political left and of “legal moralism” on the political right. Both of these factions must rely upon the coercive power of the state to force compliance with their respective philosophical goals.
Professor Randy E. Barnett expands on this idea in the paper linked below. It’s only 19 pages and well worth reading, if you wish to understand libertarianism.