Beckett often gets classed in the "Theater of the Absurd." Beckett himself disliked the label. He disliked it because it sounded like he had a thesis about life (i.e., Life is absurd). For Beckett, the whole point was that we don't know whether or not life is absurd. So, our theme is not a thesis about whether or not life is absurd. Our theme instead refers to the source of so much humor in Endgame. There are many points where the characters concerns are simply incongruous to their situation; it doesn't feel quite right. The fact that Hamm keeps his parents in trash bins, for one, is funny because it is absurd. Whether or not life is absurd, the characters seem to be saying that it certainly seems absurd right now.
Clov has a better understanding of humor than does Hamm because Clov has no ego. When one is ready to give up, everything seems humorous and absurd.
Absurdity gives the characters a rare escape from the horrible nature of their situation. In moments of physical comedy, they forget themselves and what they are going through and thus find brief moments of happiness.