Leave not a rack behind. We are such stuff (What does this set mean?)

You are really braking this quote down too much. The context of the quote should be considered in the whole quote and not fragments of it. Still, Rack denotes "a floating vapor or cloud; wisp" and reflects the relative insignificance of our existence on earth. Probably rack os a pun on the word "wreck" meaning shipwreck. In the end he is just talking about life being an illusion, the stuff of dreams.