The literal translation is: Even you, Brutus? Basically, it is the ultimate betrayal by one's closest friend. This scene, in which the conspirators in the Senate assassinate Caesar, comes after the audience has just witnessed the arrogance and hubris of Caesar, who compares himself to the gods. Brutus, a friend of Caesar who loves Rome more, has joined the conspirators in the assassination, a betrayal which is captured by the three words above.
Then Fall, Caesar means that he will fall both as a man and also as the ruler of Rome.