Not OP but gonna explain a bit about it nonetheless:
Other than the US use (generally meaning anarcho-capitalist, selfishly ignorant or both), libertarian is just the opposite pole of the “how much do we let people control us” axis from authoritarian.
So basically a left-libertarian (which is a big spectrum of different political philosophies) is broadly speaking someone who doesn’t believe in inherent authority but DOES believe in rules and various degrees of enforcement to defend the powerless from the powerful.