Context is important to understand what Macbeth means here. The Weird Sisters (witches) predicted to him in Act IV Scene 1 that he was to beware Macduff, that he could not be killed by a man of woman born, and "Macbeth shall never vanquish'd be until / Great Birnam Wood to high Dunsinane hill / Shall come against him." Because he believes the second two prophecies are impossible, he blows off the first, which seems straightforward. He thus begins to believe himself invincible.
In Act V Scene 8, however, he is confronted by Macduff, who is hot for his blood, as Macbeth has had his wife and son murdered. Macbeth tells him he's wasting his time, because "I bear a charmed life, which must not yield, / To one of woman born."
Macduff responds, "Macduff was from his mother's womb / Untimely ripp'd"--meaning he was a C-section--which understandably unsettles Macbeth a bit.
Accursed be that tongue that tells me so,
For it hath cow'd my better part of man!
And be these juggling fiends no more believed,
That palter with us in a double sense;
That keep the word of promise to our ear,
And break it to our hope. I'll not fight with thee.
He's suddenly afraid--perhaps remembering in detail the first part of the witches' prophecy, and having noted that by this point, Birnam Wood has indeed come to Dunsinane--so Macduff's words have "cow'd my better part of man!" He then turns on the Weird Sisters ("these juggling fiends"), having realized too late that they "palter with us in a double sense"--that seem straightforward but aren't--"that keep the word of promise to our ear" (that is, tell him what he wants to hear, giving him hope and confidence), "and break it to our hope," meaning that the "promises" the witches gave him are true in only one sense but dangerously untrue in another, and that other way destroys his hope, his confidence, and soon, his life.