"What is our life" is Ralegh's comment on the transience of life, similar to Shakespeare's sentiment that we are all merely players on a stage for a brief time, a metaphor repeated by Ralegh when he says that our life is "a play of passion," an allusion to medieval passion plays about the sufferings of Christ before his crucifixion.  Ralegh's tone is, of course, much more light-hearted than that of a passion play, but there is a tenuous connection.

Ralegh uses a mother's womb as a metaphor for our preparation for life--"where we are dressed"--which, unfortunately, is "this short comedy."  The concept of life as comedy is not meant to suggest that all ends happily but refers to Ralegh's belief that our lives are essentially meaningless, and certainly short, and are therefore not meant to be taken too seriously.

While we live, "Heaven," a metaphor for God, looks on and renders judgment on our behavior, particularly on those who "doth act amiss," but Ralegh has nothing meaningful to say about how we conduct our lives because he immediately moves us to the graveyard and our graves.  With some graveyard humor, Ralegh points out that graves have a useful purpose: to save us from sunburn.

The metaphor of life as drama (or comedy) is carried through the last three lines in which graves become the curtains on a stage that come down when the play is over.  All that is left for us, the players in this brief comedy, is to die "in earnest," that is, really die, not just "die" on the stage exhibiting some poor acting.

Even though Ralegh uses a common metaphor for life, a play, his tone is humorous and light throughout, which befits a comedy, and we are meant to understand that life is like a play, a diversion, that is very short and relatively insignificant.