Macbeth meant that is his destiny is to be king even if he does nothing to do such achievement. He was facing a difficult situation and he believed that destiny will do the dirty work, and he won't need to do anything for being crowned king. The original text of this famous quote in Macbeth is "If chance will have me king, why, chance may crown me without my stir" which is basically the same concept but with a different rhyme.
In this quote Macbeth uses a personification, he personifies fate. On the one hand, he wants to become a king but, on the other hand, he doesn't want to do anything about it. For this reason, he says that chance will make him a king only if he's meant to be, so that he doesn't have to try about it and so that he hasn't have to do any effort.
