The libertarian idea is guided by the Non-Aggression Principle (NAP), which states that only the initiation of force or fraud against another is immoral. Anything else is permitted, including using force or fraud to stop someone who has initiated such against another. Another way to frame it is that it's never moral to interfere with another's freedom of choice. This all stems from the idea of self-ownership: That a person is absolutely the owner and controller of their own mind and body, and that no one else can own or control them without their consent.
The NAP therefore prohibits stuff like theft, kidnapping, assault, rape, and murder. All of these actions are initiations of force or fraud, all of them are interfering with another's freedom of choice, all of them are disrespecting a person's absolute self-ownership. The NAP does not allow for common laws such as paying nonconsensual fees to the government (taxes, licenses, permits, &c.), requiring permission from the government to engage in various activities (drive a motor vehicle, engage in certain professions, develop a piece of land, hunt, fish, purchase or carry a firearm, &c.), and prohibitions against things such as drug and alcohol use, assisted suicide, or any consensual sexual activities. In fact, since all of these laws interfere with a person's freedom of choice, the laws themselves are violations of the NAP.