Strictly speaking Nihilism is not a philosophy but a denial of philosophy, a denial of the possibility of knowing, a denial that anything is valuable or meaningful. Interestingly, Nihilism is the logical conclusion of Naturalism. It is a philosophy of despair and is unlivable. After denying God's existence, the nihilist goes on to question whether the explanations go far enough. Similarly, how can we as humans claim that we are any more valuable than rocks, since we are just matter with different organization.
Morality is completely arbitrary and is therefore meaningless.
In this sense Existentialism is the answer to Nihilism, while retaining Naturalism as its base. Therefore, Existentialism reaffirms all the propositions of Naturalism except those relating to human nature and our relationship to the cosmos. Indeed its major concern is with our humanity and how we can be significant in an otherwise insignificant world. The Objective world is the domain of science, natural laws, cause and effect, death, etc. The second is the world of the Subjective where humans are free to create meaning, purpose, value, morality, etc.