Non aggression principle (will be further referred to as NAP) is a core tenet of libertarianism, stating the threat or action of physical violence against a person is inherently wrong. Note though, the NAP still recognizes violent self-defense, as in an individual using violence in response to the violence other individual or entity have done to him/her.
What is a “person”? An individual being. But the definition is also extended to include his/her private properties, and his/her right to freedom of contract which is a set of agreements that is voluntarily done by two (or more) individuals.
Given this, libertarians uses the NAP to reject things like fraud, theft, vandalism (which is violence against private property and freedom of contract) and also victimless crimes such as using drugs (since using drugs is done voluntarily by the individual and does not constitute violence against another person).
But it goes beyond that. The NAP is also the basis for rejection of the very existence of a state which has a monopoly on violence and all the policies that are derived from a state’s existence, such as taxation, military conscription, etc. where you will be forcefully punished (jail time, fines, etc.) if you choose not to comply.
Critics of NAP states that it’s definition of violence is too narrow. Under NAP, people can just hoard an entire resource and claim it as their private property (and doesn’t inflict violence to anyone in the process) and restrict its access to everyone unless they pay an unreasonably high price, which is violence (since they’re essentially starving people to death), although it is done indirectly. And to prevent these kinds of indirect violence from happening, regulations (which is directly forceful by nature) needs to be put in place. So threats of physical violence is sometimes necessary for the greater good.