Although the phrase is used in connection to Estragon's boots here, it is also later used by Vladimir with respect to his hat. Essentially it describes the hopelessness of their lives.
The play has a balance of characters, the two tramps that call out names at each other, do a little circus act by exchanging hats, try to attempt suicide, doing exercises, on the other hand the pride of Pozzo and the comic act put up by his slave Lucky. The play centers on hopelessness and despair.
A direct result of this hopelessness is the daily struggle to pass the time. Thus, most of the play is dedicated to formulating new games which will help them pass the time. This mutual yearning also addresses the question of why they stay together. Both Vladimir and Estragon admit to being happier when apart. One of the main reasons that they continue their relationship is that they need one another to pass the time.
Since passing the time is their common occupation, Estragon struggles to find games to help them accomplish their goal. Hence they engage in impertinent behaviour with one another.
Samuel Beckett has done an impeccable job of running the dialogues though it becomes easier when his characters forget everything. Estragon cannot remember anything past what was said immediately prior to his lines. Vladimir, although possessing a better...