Answer:
Macbeth is amazed and wonders if he is indeed going to become king. Banquo cautions him to be wary of the witches' words, as "oftentimes to win us to our harm/the instruments of darkness tell us truths" (1.3. 131-132). In other words, the witches may have laced their prophecy with truth in order to make Macbeth sin.
Explanation: hope this helps