When Romeo cries out, “I defy you stars” he is making reference to his love for Juliet.  In his absence from Juliet he has placed his love in the stars that both of them see each night.  This notion keeps the two of them together in thought while they are apart physically.  An abstract idea is when you explain something you cannot know with your five senses and connect it something that is known with the five senses.  You cannot see love, but you can see the stars.  The bright light of the stars on a clear night is symbolic of the love shared between Romeo and Juliet.  That is why this line is so important because it is at that moment that Romeo decides to kill himself because he has been deceived by the stars.