Final answer:
The iconic line from Shakespeare's play, Macbeth, is a spell chanted by the three witches and exemplifies the dark and supernatural elements within the play.
Step-by-step explanation:
The quote "Double, double, toil and trouble; Fire burn and cauldron bubble" from William Shakespeare's play, "Macbeth" is an example of a spell or incantation. This particular line is chanted by the three witches, also known as the Weird Sisters, and is one of the most famous lines from the play, embodying the dark and supernatural atmosphere that characterizes much of the plot. In Macbeth, Shakespeare explores themes of ambition, power, and the supernatural, and the witches' chant encapsulates the ominous tone set from the play's beginning.