In Romeo and Juliet, what does "From ancient grudge break to new mutiny" mean?

In the prologue of the play, the Chorus describes what is going to happen in a short summary of events, including that the play is going to end with the deaths of two lovers.  In part, they say, "From ancient grudge break to new mutiny, / Where civil blood makes civil hands unclean."  In so saying, the Chorus refers to the feud between two respected families of Verona, the city where this play takes place; the families' grudge against each other is old and has lasted quite a long time.  Further, in these lines, the Chorus says that this ancient feud will erupt in new violence, violence that will result in the spilling of blood and the shared guilt of both families.  The Chorus goes on to say that the children of these families will fall in love and kill themselves, and peace will only come to exist once these families' greatest joys -- their children -- are gone.