The witches are the ones who give Macbeth the prophecy that he will one day become king, that Banquo with father kings, and that Macbeth can not be killed by anyone born of woman. These set off Macbeth's entire journey in the play and motivate him to kill the king, kill Banquo, and ultimately lead to his downfall.
It means that what is that set of beliefs and it also tells us that is not important how important it is, In other words it importance doesn't matter.