Absurdism is a type of philosophy. People who support and argue for absurdism are known as 'absurdists'. Absurdists think that the human condition is essentially absurd because humans are always looking for meaning, and also completely unable to find meaning. In philosophy, this is called "the Absurd". In this case absurd means that it is not possible to be done by humans.
Absurdists, most notably Albert Camus, believe that when human beings realise this fundamental absurdity they have different reactions. One reaction is suicide, because they feel that the only solution to an absurd life is to stop living. Another reaction is to believe in something higher (Camus gave religion as an example of this), when they see that there is no meaning they can find in the universe (which is what absurdism says is the basic state of human experience). The final reaction is to accept the absurd, and also to keep trying to overcome it. Camus believed that a human being could become happy by finding meaning in their relationship with the absurdity of their existence.