Libertarianism has roots in leftism, actually, the Capitalists appropriated the word.
Either way, Anarcho-Capitalism is an ideology surrounding the magical idea that Capitalism can exist without a state. The delusional people that follow such an ideology like to imagine that security forces would be paid for, and somehow be better than public forces, in order to uphold the legitimacy of Private Property. They also foolishly believe that monopoly wouldn't happen because there isn't a state to prop them up.
AnCaps are just edgier libertarian Capitalists with even less common sense, surprisingly.