It refers to someone who advocates:
Obviously, there's more to it.
What is freedom? It is living unconstrained by other people's impositions, it is an absence of constraints proactively imposed personally on you by others. Perhaps the best way to see freedom is to remember its symmetric opposite: Being proactively imprisoned in a cage against your will. A libertarian wants to maximise human freedom.
What is voluntary human interaction? It is all our interaction with other people that we voluntarily choose to engage in. It includes cooperation, competition, community, conversation, sport, organisation, relationship, trade, and so on. The opposite of voluntary human interaction is aggression. A libertarian wants to maximise voluntary interaction and minimise aggression.
What is anti-statism? It is being opposed to the State, what most today call "the government." The social science definition of the State is a territorial monopoly on mass ideologically legitimised, mass coercion. For example, if Jane chooses not to pay taxes for a war, she will eventually have men come to her door and force her into a cage at gunpoint. However most people assume this coercion is "legitimate." Thus, the State violates human freedom and every act of State is an act of interpersonal aggression. A libertarian wants to minimise the State, ideally abolishing it altogether.
Why are libertarians libertarians? Many diverse reasons. We argue that human freedom is precious in itself. We argue voluntary interaction has virtue on its own. We argue more libertarian, non-state societies empirically do and logically will result in the elimination of poverty, greater effective equality, more real community, the end of starvation, faster technological innovation, widespread prosperity, economic growth, more optimal environmental outcomes, more efficient and equitable law, greater human creativity, happiness, and well-being.