In this speech, Pozzo is expressing his frustration about the concept of time and the passage of time.  When Pozzo first appears in the play he is able to see and has a servant named Lucky who he has on a leash.  In this scene, the tables have turned a bit and Pozzo is now blind and he has the leash attached to Lucky so that he, himself, doesn't run into things or get lost.  Vladimir keeps asking about the passage of time and when this happened to him.  Pozzo doesn't know the answer the question and is annoyed by the whole situation, so in his extreme anger and frustration says

Have you not done tormenting me with your accursed time!  It's abominable!  When! When! One day, is that not enough for you, one day he went dumb, one day I went blind, one day we'll go deaf, one day we were born, one day we shall die, the same day, the same second, is that not enough for you? 

He is annoyed that Vladimir isn't accepting of a vague generalized answer like "one day..." he wants to know the exact day.  This all leads to the final line of the speech

They give birth astride a grave, the light gleams an instant, then it's night once more. 

Here he is making a generalization of the fate of all mankind.  The quote means that birth is the beginning of death.  Women give birth astride a grave suggests that life is only a brief flash of time "an instant" and then death is the inevitable end.  Time is ultimately rather meaningless when ones whole life time is condensed into a brief moment.  It is an incredibly pessimistic attitude, yet that suits Pozzo's personality, especially considering the circumstances of his life in Act 2.