In Act I, Scene 3 of Macbeth, Banquo and Macbeth are just riding in from the battle field, leading their troops behind them. They dismount and are walking alone to see King Duncan after winning a successful battle against the traitorous Thane of Cawdor. On the way, Banquo and Macbeth come upon the three witches who have been preparing and waiting for Macbeth.
The witches, once they are bidden by Macbeth to break their silence and speak, give prophecies to Banquo and Macbeth. Banquo will be the father of kings and Macbeth will be Thane of Cawdor, who is at the moment held as a prisoner, and will be King. Macbeth is startled at the news.
Momentarily, riders from the King approach and tell Macbeth that the title of Thane of Cawdor has been bestowed on him because the traitor will be executed. He then asks Banquo if he believes the witches now because of the deliverance of their Cawdor prophecy. This is where Banquo administers his warning, saying that sometimes evil works by trickery. He means that evil may tell truths and grant trifles in order to lure the innocent into great consequences of wrong and evil acts in order to have the greater promise that they now lust for. Banquo spoke a true warning to Macbeth as events later reveal.
That trusted home
Might yet enkindle you unto the crown,
Besides the Thane of Cawdor. 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. (Act I, Scene 3, Lines 120-126)