This question seems to be referring to Banquo's aside to Macbeth, which, according to the e-text of Macbeth offered by this site, encompasses lines 130-135. Macbeth and Banquo have just discovered that the Thane of Cawdor, having committed treason against the King, is to be put to death, and that Macbeth will assume his title. This, of course, is what the two men had just heard the witches prophesy earlier in the scene, and Macbeth is taken aback by this knowledge. In the passage in question, Banquo says that this knowledge may "enkindle you unto the crown," in other words, cause him to begin to believe that the second part of the prophecy--that he will become king--might come true as well. But the second part of the speech is the most important:
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...
By this he seems to be warning that though the witches may be telling the truth, their motives in doing so are still suspect. They may have told Macbeth these "honest trifles" in order to manipulate, or lead him to do horrible things to make them come true. This speech by Banquo turns out to be as prophetic as anything the witches say.