Shakespeare's plays are typically written in blank verse (unrhymed iambic pentameter). The witches, however, speak in a type of trochaic tetrameter and rhyming couplets. This meter begins with an accented syllable; tetrameter means there are four units of meter.
Here are some examples (accented syllables are capitalized):
THEN the CHARM is FIRM and GOOD
SCALE of DRAgon, TOOTH of WOLF
What's unusual about the witches' lines is that they are not perfect trochaic meter. Each line lacks an unaccented syllable because the verse ends on an accented syllable; rather than having eight syllables in a line, most verses have only seven.
The Elizabethan audiences would have been delighted to see the witches, which they very much believed really existed, and they would have quickly discerned the difference in the way the witches spoke. Their lines immediately set them apart from the other characters, both nobility and commoners.
Shakespeare relies on alliteration, rhyme, rhythm, and repetition in this scene. Here is an explanation for each.
Alliteration - repetition of initial consonant sound. Alliteration is the verbal equivalent of a drum beat, adding to the sense of rhythm that may or may not already exist in a passage. Here is an example: "toil and trouble" and also "boil and bubble."
Rhyme - the use of rhyme creates a song-like effect to a piece of writing. This elevates the writing, making it some cases more romantic, in others more suspenseful. In this case, it is suspenseful, accentuating and connecting each line to the other and - again - acting like a drum beat in the scene. The rhyme scheme is aabb, meaning there are always two lines together that rhyme.
Rhythm - This is the versification of written language, so that the "beat" of the words is the same and repeats. In this case, the rhythm is in iambic pentameter, meaning that the beat is "duh DA" and it is carried for five feet (measures) per line. The beat builds the tension in the theme.
Repetition - Each of the listed techniques is a form of repetition. However, Shakespeare also repeats the chorus of "Double, double toil and trouble; / Fire burn and cauldron bubble" (VI.i). This underscores the idea of a "spell" and makes the scene more creepy.
All Witches: Double, double toil and trouble;
Fire burn and cauldron bubble. (IV.i)
Just so you know, many scholars believe that Shakespeare did not write this scene. It was a spin off and sometimes directly quoted lines from The Witch, a play by Thomas Middleton, written around the same time. The poetic devices are actually evidence that Shakespeare did not write this scene. Throughout Macbeth, Shakespeare uses iambic pentameter, as opposed to the trochaic pentameter used by the witches, although it is not perfect. This further provides evidence that Shakespeare did not write this scene, since Shakespeare never switched his styles of writing so dramatically nor so poorly. The quality of the writing is notably poorer, and I find it hard to believe that it was Shakespeare.