Shall I put this in plain English?
Some time ago, there was a movement called Modernism (I'm sketching it out here - don't expect rigour, please). It was all part of the early twentieth century excitement about the speed and pace and drama of industrial society, and it placed a heavy value on 'newness.' In its most brutal incarnations, this movement got all aroused by War (hello, futurists), but it generally had a hard-on for progress.
It is where all the contemporary chat about how brilliant science is comes from. It fits it quite nicely with nationalist politics, too: as nation states tried to define themselves after the collapse of the Turkish, British and French Empires, the idea of progress was hip. Besides, things were looking up: medical science, technology, diplomacy: the old days of a life brutish and short were on their way out.
It did mean ignoring the continued class war on the poor, and the exclusion of many African and Asian nations, but still...
The Second World War punched a hole in modernist optimism. It took a while for everyone to release, but defeating the Nazis was not as good as not having Nazis in the first place. Human nature, which had been looking tasty and progressive, was exposed as a shit-pit of racism and horror. Then the nuclear bomb got dropped, and the very air was contaminated.
The absurdists - Beckett, Pinter, Ionesco - sensed something was up with these post-enlightenment dreams: academic Carl Lavery has suggested (in not so many words) that they were shitting it about the state of the environment. The meaninglessness in their works - and the later emphasis by post-modern thinkers on refusing any single story or belief that could make sense of the world - was not so much tricky writers being vague: they were presenting a truth that challenged the notion of security, and thence the notion of truth itself.
Even trying to describe that is a head-fuck.
In exploring these ideas, theatre got experimental. Certain actions became associated with experimentation.
In the next episode, we ask what happened next...