Absurdism is a kind of philosophy. Proponents of absurdism are known as 'absurdists', and they hold that there is an essential absurdity in the nature of the human condition - that we are both constantly looking for meaning, and also completely unable to find it.
Absurdists, most notably Albert Camus, believe that there are a number of reactions that human beings have when they realise this fundamental absurdity. One is suicide - which is when a human being sees the only solution to the problem of their absurd existence as putting an end to their experience of it. The next reaction is the taking up of faith in some higher meaning (Camus gave religion as an example of this), in the face of a lack of any apparent meaning in the universe (which is what absurdism holds is the basic state of human experience). The final reaction is an acceptance of the absurd, coupled with a continuing effort to attempt to surmount it. Camus believed that a human being could achieve happiness by finding meaning in their relationship with the absurdity of their existence.