What does Juliet mean when she says "what must be shall be?"

Juliet is referring to her upcoming marriage to Paris, something that will never happen. In essence, her words have double meaning....... to appease Paris, and yet, tell the truth.

She has firmly decided to accept her destiny. In the rollercoaster ride of emotions, fate is in her way, and there is no other road "highway" than to walk into what may be her final steps in life.