what is the background to the witches giving Macbeth the prophecy

The witches (commonly referred to as the Weird Sisters) prophesy that Macbeth will be king of Scotland in the first few scenes of the play. The play itself begins at the end of a battle in which Macbeth has led the forces of Scotland to victory over Macdonwald on a gloomy plain (or "blasted heath," as it's often called).

Although the reader is given little information about who these Weird Sisters are and where they've...

The witches (commonly referred to as the Weird Sisters) prophesy that Macbeth will be king of Scotland in the first few scenes of the play. The play itself begins at the end of a battle in which Macbeth has led the forces of Scotland to victory over Macdonwald on a gloomy plain (or "blasted heath," as it's often called).

Although the reader is given little information about who these Weird Sisters are and where they've come from, their presence in the play is absolutely essential, and so their origins and purpose are worth thinking about. According to the Encyclopedia Britannica, the term "Weird Sisters" was first used by Scottish writers to refer to the Fates, the group of women responsible for deciding the destinies of mortals in Greek and Roman mythology. This context is important, as the Weird Sisters essentially act as the Fates in a Scottish, medieval setting. Their prophesy determines not only the fate of Macbeth and his wife, but also the fate of the kingdom of Scotland. And what a gruesome fate it is.  

It's also worth considering the overall purpose of the Sisters in the play. Overall, they function as the primary supernatural presence in Shakespeare's dark reimagining of Scotland, and they immediately signify that the fabric separating reality from the spirit world (or sanity from insanity, if you prefer to think of it in those terms) will be very thin indeed. Additionally, they lend the play an eerie, often downright creepy tone that perfectly sets up Macbeth's tragic downfall. All in all, it's possible to read the Sisters as a personification of fate and destiny, but there are few other clues to their exact origins. This mystery of course only tempts readers to ask further questions about them.