word fair during the Renaissance usually means beautiful; and foul is the opposite, meaning ugly. The witches are presenting a riddle that will take on significance as the play progresses. The way to interpret this line is as follows: what appears to be beautiful is actually ugly, and what appears to be ugly will actually be beautiful. This line thus foreshadows the predictions that they will make for Macbeth. The fair or beautiful words (or prophecies) that they speak to Macbeth will actually turn out to be ugly or even evil. The witches are not to be trusted then. They will be misleading or tricking Macbeth into actions that will bring about his own harm.