The witches in the story, Macbeth, had three prophecies. 1st, Macbeth will be name Thane of Cawdor. 2nd, Macbeth will become King of Scotland. 3rd, the witches prophesied that Banquo will be the father of a long line of Scotland's future kings.
After these prophecies, Ross showed up and made the 1st prophecy true by claiming that Macbeth has become the Thane of Cawdor. This fulfillment of the 1st prophecy provided evidence that the other prophecies of the witches will come true.
