(Actually it's 
Our mothers' wombs the tiring-houses be 
Where we are dressed for this short comedy"
but to the point:)
The poet compares a womans' womb to a tiring-house: a place where an actor would get ready for a scene and await their cue to perform. In his mind all people are like these actors; we spend nine months getting ready to be born and then our time comes to get on the methaporical stage of life. We interact with other people (participants of the play); we show our skills, grow and learn like a well-writting character should.  
But our time on the stage is short. Death marks the end of our performance, and the play turns out to be a comedy- whether because of the pathetic content: pointless struggles agains life's obstacles, or because of its laughable brevity.
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