What warning does Banquo give Macbeth?

In Act I, Scene 3, after Macbeth and Banquo have heard the Witches' prophecies, Macbeth asks Banquo:

Do you not hope your children shall be kings,When those that gave the Thane of Cawdor to mePromised no less to them?

What Macbeth means here is that the Witches promised that he would become king and also promised that Banquo's heirs would be kings. In response, Banquo gives Macbeth the following warning:

But ’tis strange;...

In Act I, Scene 3, after Macbeth and Banquo have heard the Witches' prophecies, Macbeth asks Banquo:

Do you not hope your children shall be kings,
When those that gave the Thane of Cawdor to me
Promised no less to them?

What Macbeth means here is that the Witches promised that he would become king and also promised that Banquo's heirs would be kings. In response, Banquo gives Macbeth the following warning:

But ’tis strange;
And oftentimes, to win us to our harm,
The instruments of darkness tell us truths,
Win us with honest trifles, to betray's
In deepest consequence—

This suggests that the Witches, who are instruments of darkness, agents of the Devil, are not there to do either Macbeth or Banquo any good but to do them harm. Banquo's premonition turns out to be true because both men are destroyed by the play's end. Macbeth and Banquo quickly become suspicious of each other. Macbeth strikes first. He recruits men to murder both Banquo and his son Fleance--although Fleance manages to escape and keeps the prospect of Banquo's heirs becoming kings still alive. Macbeth murders King Duncan and manages to supplant him as king of Scotland. But the thanes and the common people hate him because they know he is a murderer and a usurper. He has to rule by escalating force and fear, leading to domestic chaos and desertions. The English king sends an army headed by Duncan's son and legitimate heir along with Macduff, the Thane of Fife. Macbeth trusts the "instruments of darkness" until the end, when he is slain by Macduff in hand-to-hand combat and his head is displayed on a pike.

So Shakespeare, through Banquo, is suggesting that the Three Witches stop Macbeth and Banquo on the heath in order to plant false notions in both their heads and cause their destruction with real truths that have fatal consequences. Macbeth tries to make the prophecies come true in a proactive manner, but Banquo keeps his head and does nothing. He reasons that if Fate intends for certain things to happen to him and his heirs, then those things will happen without his trying to interfere with Fate. Otherwise he will be content with whatever happens.