A character’s way of speaking tells us something about that character
Whether a character in Macbeth speaks in prose, verse, or rhyme is important because the specific language can reflect the character’s social status or the character’s state of mind.
The style of language in Shakespeare’s plays is mainly a mixture of prose and verse. Prose is what we would call ordinary language, whereas verse is characterized by a specific metrical rhythm (although the meter in Macbeth is often fairly irregular). Sometimes verse includes rhyme.
Generally, prose can be used as a sign of very different things. Prose is considered less “posh”, which is why it is often used to signal low social status. This is the reason we see non-nobles or commoners speak this way in Macbeth.
Prose may also be used for comical purposes. For example, in Act 2, Scene 3, the hung-over working-class porter speaks in prose.
We also typically find prose in letters. An example of this is Macbeth’s letter to his wife in Act 1, Scene 5.