It is important to see the entire play as a process. In this very important scene we see that Macbeth is completely taken back by the prophecy of the witches and also the way that partly it has been instantly fulfilled by being made the Thane of Cawdor. As he dwells on the truth of the prophecy with this regard, he then thinks about the rest of the prophecy and if he will need to do anything himself to make it happen. He finally concludes the following in an aside:
If Chance will have me King, why, Chance may crown me,
Without my stir.
He basically thinks that if it is decreed that he should be King, it will happen without him having to do anything about it. This is important to note at this stage in the play, because it shows Macbeth at the beginning of his downward plummet towards evil. He has logically concluded that if it is prophesied that he will be King, then he doesn't have to act to help destiny in its course. Waiting and seeing what "Chance" has for him shows that he has not given himself over to evil by lending "Chance" a helping hand and killing Duncan himself.