In Waiting for Godot, Vladimir and Estragon wait because they think there is some purpose to waiting. They think that Godot will bring them some good news, some important bit of information, anything that will give them some sense of assurance or certainty about what to do or about why they have been waiting in the first place.
While they wait, nothing essentially happens. Their speech is chaotic, terse, forgetful, sometimes philosophical but only to question why things are so uncertain. They consider suicide as an alternative to waiting; if Godot isn't coming (the only certainty they can suppose exists) then what is the point of living?
The intrusions by the bully, Pozzo, and his slave, Lucky, are random, violent and strange. When Vladimir and Estragon find themselves waiting again the next day, the tree has changed. Nothing is certain. Nothing makes sense. And yet, despite living in this random, uncertain, seemingly meaningless world, they continue to wait, expecting that Godot will provide some sense or meaning. And they continue to wait despite Godot's failure to appear day after day. This is the definition of absurdity. To be in an uncertain or meaningless situation or existence and to still expect to find certainty and meaning.