In Shakespeare's Macbeth, the second witch says in scene one that the three will meet again:
When the hurlyburly's done,
When the battle's lost and won.
The hurlyburly is the battle. She also says they will meet upon the heath, or field, and the third witch adds that they will be there to "meet with Macbeth."
As is obvious, of course, the witches ignite the action of the play with their predictions to Macbeth. Another issue exists, however: how do they know where Macbeth will be, and how do they know which side is going to win the battle? If the rebels were to win, there would be no reason to meet with Macbeth, since he would then be in no position to gain power of any kind, having fought on the losing side.
Depending on how you interpret the play, the witches are just very well informed, or they supernaturally know the outcome of the battle. Duncan seems very worried about the outcome, so if the witches are not supernatural, they know something Duncan doesn't, which is a bit hard to accept.
This contributes to the issues of the unnatural, the supernatural, predestination and free will (fate), and Macbeth's responsibilty, all present in the tragedy.
I assume that you are talking about what the witches are planning in Act I, Scene 1. You can find the answer to this in the first ten lines of the play. The witches are planning to meet up with Macbeth.
The play opens with the witches, which is appropriate since they drive so much of the action. When they meet Macbeth, as they are planning, they will give to him the prophecy that will drive all the action of the play. They will tell him he is destined to become king.
The witches plan to meet Macbeth in Act 1 after the battle is over. Macbeth along with Banquo was coming winning the battle and the wicked 'weird sisters' prepare to meet him on their way. That is the beginning of the play and the starting of Macbeth's doom. The witches would spell some prophesies and those would instigate his inner ambition.
And this is clearly evident in the third witch's speech: "There to meet with Macbeth." in response to the first witch's question: "When shall we three meet again?" and "Where the place?"