In Romeo and Juliet, what is meant by the phrase "from forth the fatal loins of these two foes"?

The phrase, mentioned in line five of the Prologue, sets the stage for the action yet to come.  The "loins" refer to the sexual organs of both sets of parents; even in modern society, we sometimes refer to children as the "fruits of our loins."  Thus, those who came "from the loins" would be Romeo and Juliet.  Calling the loins "fatal" helps the audience to understand that things will not end well for Romeo and Juliet; both will perish by the end of the story.  The "foes" referred to within the line are the Montagues and the Capulets, two families that have been at war for a considerable amount of time.

In general, this line fits into a prologue that details the full action of the play before the action even begins.  Some might question giving away the ending during the prologue, but doing so provides its own method of suspense.