theatre of the absurd originated from existentialist thoughts. Existentialists tried to pose the question of "why are we alive?" Theorists believe that we do 'nothing' on earth and we wait until we die. When we die, then we become alive. It's really about a pointless existence.
Theatre of the absurd originated from these ideas. So the themes of absurd theatre are the "nothingness of life" and "pointless existences".
Is this for drama? Coz there are some great links for existentialism/theatre of the absurd in the drama forum.