In this scene, Romeo has been banished from Verona for killing Tybalt, Juliet's cousin. He has come to Juliet's chamber to say goodbye before he leaves, and they are both distraught over the situation. Romeo feels like he is being controlled by fate, or "fortune," and that he has no control over his own life or happiness.
The phrase "I am fortune's fool" can also be interpreted as a reference to the ancient Roman concept of Fortuna, the goddess of fortune. Fortuna was often depicted as a blindfolded woman standing on a wheel, symbolizing the unpredictability and capriciousness of fate. Romeo feels that he is at the mercy of this blind goddess, and that he is being punished for some unknown reason.
Romeo's use of the word "fool" is also significant. In Shakespeare's time, the word "fool" was often used to refer to someone who was silly, gullible, or foolish. However, it could also be used in a more serious way to refer to someone who was unfortunate or unlucky. In this case, Romeo is using the word "fool" to describe himself as someone who has been unlucky in love and who has been dealt a cruel fate.
Overall, the phrase "O, I am fortune's fool!" expresses Romeo's deep sense of despair and his feeling of powerlessness in the face of fate. It is a powerful statement that captures the tragic nature of Romeo and Juliet's love story.