What does the line the "fearful passage of their death-mark'd love" mean in Romeo and Juliet?

This line means the play will tell us about Romeo and Juliet’s doomed love.

The quote "the fearful passage of their death-mark'd love" means the play is about Romeo and Juliet’s doomed story. Their love is doomed. We are told this ahead of time to make sure we appreciate the full arc of the story.

And the continuance of their parents' rage,

Which, but their children’s end, naught could remove,

Is now the two hours' traffic of our stage (Prologue).

Most people now know Romeo and Juliet's story. Although their families are feuding, the teenagers fell in love at first sight. They married in secret, deciding their love was more important than their parents' hate.

That's all fine and good, but like most teenagers, Romeo and Juliet failed to see the bigger picture. They went to Friar Laurence to marry them in secret, and he agreed because he thought it would help end the fighting between their families.

It didn't work. Romeo and Juliet might have loved each other, but hate was strong in their families. Juliet's cousin Tybalt picked a fight with Romeo. His friend Mercutio intervened, and Tybalt killed him. Romeo then killed Tybalt, his own new bride's cousin. He was banished.

From there, the story is history—or tragedy. Juliet is being forced to marry someone else, but can't because she is married to Romeo. She goes to Friar Laurence, who helps her fake her death. Romeo doesn't know, and he thinks Juliet is dead. He kills himself. Juliet wakes to find him dead, and kills herself. That is the tragedy the prologue foreshadows.

The prologue also tells us Romeo and Juliet did not die in vain. After their young children die, the Capulets and Montagues realize their feud was ill-advised. They decide to end it, so some good comes of the tragic loss of two young lives.