What does Macbeth say he and Banquo will talk about later in private in Macbeth?

Macbeth tells Banquo that they will discuss the prophecies later.

When Macbeth meets the three witches for the first time, he is not alone. He is accompanied by his friend and fellow noble, Banquo. Banquo is immediately suspicious about the whole thing. He would rather Macbeth just forget he ever heard the prophecies. One applies to Banquo too. He is told that his sons will be king.

Macbeth does not let it go. When he was told that he would become Thane of Cawdor and king, he became very interested. When King Duncan appointed his son Malcolm as his successor, Macbeth was irritated. He had a spot of ambition and wanted the position for himself. He told no one, though, except his wife, Lady Macbeth. He wrote her a letter.

Macbeth invites Duncan to his house, and the other nobles are there too. Banquo is up late at night talking to his son when Macbeth comes to him. Banquo is surprised to see him, wondering why he is not in bed. Banquo comments that he has been thinking about the prophecies.

BANQUO

All's well.
I dreamt last night of the three weird sisters:
To you they have show'd some truth.

MACBETH

I think not of them:
Yet, when we can entreat an hour to serve,
We would spend it in some words upon that business,
If you would grant the time. (Act 2, Scene 1)

When Macbeth says that he has not thought about the witches, he is lying through his teeth. He has been thinking about them almost constantly. He wants to kill Duncan and take his place on the throne. His wife wants this too. Yet Banquo is an impediment to the plan because he was there, and heard the “weird sisters” make their prediction. Macbeth has to make Banquo think he does not care, so that he will not be suspicious when Duncan ends up dead.

It doesn’t work. Banquo knows immediately that something is up when Duncan dies. He knows Macbeth too well to think it is a coincidence. Banquo ponders the situation. 

Thou hast it now: king, Cawdor, Glamis, all,
As the weird women promised, and, I fear,
Thou play'dst most foully for't: yet it was said
It should not stand in thy posterity,
But that myself should be the root and father
Of many kings. (Act 3, Scene 1)

He is speaking to Macbeth, in a soliloquy (in other words, Macbeth is not really there). This is known as apostrophe, a literary term for speaking to someone who is not actually present. Banquo believes that Macbeth killed Duncan to become king. He considers how this prophecy might affect himself. Since the witches suggested that his sons would be king, Banquo worries that he and his son Fleance are in danger.

Banquo was right. Macbeth hires murderers to kill Banquo and Fleance. Fleance escapes, but Banquo is killed. He was the greatest danger to Macbeth, since he knew about the prophecies. Once you have done one murder, the rest get easier! Macbeth is nowhere near done killing.