Nihilism is the philosophical position that there are no objective moral truths, no inherent meaning in life, no values that exist apart from the people who hold them. Stated baldly, it sounds devastating. In practice, most thoughtful nihilists treat it as a starting point rather than a verdict.
Nietzsche, who took the diagnosis most seriously, distinguished passive nihilism (the despair that follows the collapse of inherited meaning) from active nihilism (the project of building new values once you accept that no values are given). His "death of god" wasn't a celebration — it was an alarm. If we can't honestly believe what we used to believe, what do we do next?
Twentieth-century existentialists (Sartre, Camus) lived in this aftermath. Camus's question — "is life worth living?" — assumes nihilism as the backdrop and then refuses to let the answer be no. His response was the absurd hero, who keeps going without metaphysical comfort, knowing the meaninglessness and choosing meaning anyway.
Nihilism gets a bad reputation because it's confused with apathy. Real nihilism is the opposite — it's the lucid recognition that nothing has been settled for you, which means everything is genuinely at stake in how you choose to live.