According to Holman and Harmon, the term stands for "the kind of drama that presents a view of the absurdity of the human condition by the abandoning of usual or rational devices and by the use of nonrealistic form."
Typically, there is not a series of events that tells a story, but rather a pattern of images which present people as bewildered beings in an incomprehensible universe. This is a pretty good description of what happens in "Waiting for Godot". Nothing happens. They sit and talk about things--nothing in particular--and wait for Godot who never arrives. The reader who is careful and attentive will laugh hysterically as he reads the play since there is wild humor in not only the situation but also the ridiculousness of the conversation between the two who wait.
Check the link below for more explanation and examples from the play which illustrate the term "theater of the absurd".
Waiting for Godot certainly includes absurd elements in its lack of real plot, and its premise that the universe has no meaning and is, in itself, irrational. It also shows traits of the absurd in its inclusion of existentialist topics, such as alluding to the self-conscious and our inherent natural freedom to explore the carpe diem.
It breaks away from the traditional methods and literary devices for narrative, lacks specific stage directions, leaving it amorphous and "up for grabs", and finally, the end is never quite reached. A plot usually has a clear cause and effect of the rising action, but here the characters continue doing their exploratory of the "self" without really seeing Godot.
All absurdist plays are characterized for the chaotic organization, for its exploration of existential nature of man, of the self, of the conscious, and of life as a mystery.
Waiting for Godot revolutionized theatre in the twentieth century and had a profound influence on generations of succeeding dramatists, including such renowned contemporary playwrights as Harold Pinter and Tom Stoppard. Initially written in French in 1948 as En Attendant Godot, Beckett's play was published in French, later translated into English by Beckett himself as Waiting for Godot. Beckett's play came to be considered an essential example of what Martin Esslin later called "Theatre of the Absurd," a term that Beckett disavowed ."Absurdist Theatre" discards traditional plot, characters, and action to assault its audience with a disorienting experience. Characters often engage in seemingly meaningless dialogue or activities, and, as a result, the audience senses what it is like to live in a universe that doesn't "make sense." Beckett and others who adopted this style felt that this disoriented feeling was a more honest response to the post World War II world than the traditional belief in a rationally ordered universe. Waiting for Godot like Ionesco's Amédée remains the most famous example of this form of drama. Beckett often focused on the idea of "the suffering of being." Most of the play deals with the fact that Estragon and Vladimir are waiting for something to alleviate their boredom. Godot can be understood as one of the many things in life that people wait for. Waiting for Godot is part of the ‘Theater of the Absurd’. This implies that it is meant to be irrational. Absurd theater does away with the concepts of drama, chronological plot, logical language, themes, and recognizable settings. There is also a split between the intellect and the body within the work. Thus Vladimir represents the intellect and Estragon the body, both of whom cannot exist without the other. Albee's Zoo Story is clearly far more firmly anchored in reality. Adamov's Professor Taranne came as a dream. But Beckett’s Waiting For Godot stands somewhere between dream and reality. In Waiting for Godot for example Beckett parodies and mocks the language of philosophy and science in Lucky's famous speech. Harold Pinter, in his The Caretaker shows uncanny accuracy in the reproduction of real conversation among English people. In its critique of language the Theatre of the Absurd closely reflects the preoccupation of contemporary philosophy with language, its effort to disentangle language, as a genuine instrument for logic and the discovery of reality, from the welter of emotive, illogical usages. the Theatre of the Absurd has much in common with the existential philosophy of Heidegger, Sartre, and Camus. It was in fact Camus who coined the concept of the Absurd in the sense in which it is used in Waiting for Godot. The play has often been viewed as fundamentally existentialist in its take on life. The fact that none of the characters retain a clear mental history means that they are constantly struggling to prove their existence. Thus the boy who consistently fails to remember either of the two protagonists casts doubt on their very existence. This is why Vladimir demands to know that the boy will in fact remember them the next day. According to Holman and Harmon, the term ‘absurd drama’ stands for "the kind of drama that presents a view of the absurdity of the human condition by the abandoning of usual or rational devices and by the use of nonrealistic form." This is true of Waiting for Godot