Macbeth actually opens with this very question. After asking when they should meet again in the first line of the play, the witches agree to convene at the end of a great battle:
When the hurlyburly's done;
When the battle's lost and won.
This will be, they say, before the end of the day, and they announce that they will meet "upon the heath" where they will encounter Macbeth. The audience is not informed as to who Macbeth actually is, but it can be inferred that he is involved in the battle. The scene is obviously designed to set the plot in motion, but also to create a certain mood. The theme of opposites ("lost and won") is pursued throughout the scene, suggesting the disconnect between appearances and reality that we see throughout the play. It is clear that the witches have nothing but malevolent plans in store for Macbeth.