To be universal, a play must relate to the human condition, in any situation. This means that it has themes that can address the audience no matter their situation in life (rich, poor, male, female, etc.).
Most, if not all literature, has universal elements. In this play, however, Beckett goes out of his way to make the message as broad as possible. He does this by leaving many details out. No information is given about the characters background, nationality, or economic status. Their description and demeanor suggests that they were once respectable, but they are now homeless. However, they do not appear to be suffering as a result of their poverty. Therefore, they could represent a wide range of people.
In addition, the few characters that are included cover a range of personalities. Vladimir is compassionate, assertive, and intelligent; Estragon is emotional and needy; Lucky is subservient, but obvioulsy educated; Pogo is demanding and arrogant.
The conflict of Godot - choosing to wait for something that does not seem to be possible - is vague enough to be interpreted many different ways. The men could be waiting for faith/God, waiting to improve their life, waiting for opportunity, for adventure, for love.... the list goes on.
The vague details of the play allow it to be experienced in any number of ways, adding to its universal characteristics.