Vladimir and Estragon are waiting for someone who presumably will never show up. They receive a message that he is not coming but will show up the next day. But given the repetition of waiting and uncertainty, it is unlikely that Godot will ever show up. Tied in with the themes of memory and time is the theme of uncertainty. Everything is uncertain in the play and this is what makes it appear absurd.
Consider the play as an allegory of a chaotic modern world, one in which religion, politics, and progress provide no structured or logical sense of humanity's place in the world. Or consider it on a more individual level. We have two characters who are existentially lost. Thinking in broad historical terms or on the personal level, they have knowledge (memory) of religious beliefs, of philosophical truths, of systems of thinking. And yet, none of these historical/personal memories provide them with a sense of what it means to be human any longer. These memories (long and short term) don't help them understand their place in the world anymore.
Note that the language in the play is often so playful that it ceases to make logical/linear sense. Even their most basic memories of language itself are clouded with uncertainties. And there are points in the play when even their short-term memories fail them. Early in the play, they argue about what they did the day before and if they are in the proper waiting area.
Everything they've learned no longer provides them with answers. Anything dealing with the meaning of living, of life (from religion to philosophy to daily material human endeavors) has ceased to give them a sense of logical meaning. Thus, memory has failed them. So, they are left with waiting for something or someone to come and make sense of it all. And they wait, over and over, time becoming redundant and cyclical. Adding to the absurdity, time becomes meaningless for them, but is the only thing they have to rely on. If time is cyclical and redundant, there is no progress. Historically speaking, if there is no progress, we don't grow, learn, and get to a better, wiser place. This is one interpretation of the play in terms of historical progress: there is no progress, or progress has stopped. Waiting for someone or for some grand informative memory to return is a waste of time, existentially speaking. The moral, in this context, is to stop relying on memory, stop waiting, and to start coming up with something new. This could break the redundant cycle of time that comes with a lack of progress.