Existentialism is the philosophy of, well, existence. It states that life and existence is without purpose. Life, the universe, and everything is void of any purpose or meaning. Existentialism rejects the use of reason to define some meaning, as it is an attempt to impose a sort of order in the world of phenomena, which is essentially irration, unscrutured, and random. Rationality hinders us from finding meaning--it's "bad faith". To try and suppress our anxiety and dread that we feel in our own freedom and our awareness of death, we confine ourselves in the everyday experience, in which we give up our freedom. In Existentialism, people make decisions based on their interpretation of meaning rather than what's rational. The individual longs for an order, and this conflicts with the world of phenomena's randomness. This means the universe, where people try to be rational in an irrational world, is absurd.
How do we, then, find meaning? In Existentialism, meaning is not provided by some divine order, but rather is created by the individual's actions and interpretations. In the absurdity of the universe, where the universe has you in a void of nothingness and you're feel like you're just thrown into existence, meaning is sought through embracing existence, the individual creates the meaning.
"Existence Precedes Essence"