The short scene prepares the audience for the imminent battle. Malcolm has forged a military alliance with the King of England and rebelling Scots. Just as the third apparition had predicted in Act IV Scene I, Birnam is an important an important ingredient to Macbeth's downfall. As Caithness notes, Macbeth’s title “hands loose about him, like a giant's robe upon a dwarfish thief.” Macbeth’s reign verges on collapse.
In Act 1, it is notable in Scene 3 that Banquo referred to Macbeth’s honors as strange titles. Macbeth had attained his position fraudulently, and no longer can he sustain it. He has shed so much blood that his presence in power will curse Scotland if he retains it. The rebelling Scots sound confident in the outcome of their efforts. They believe that time has come for them to bring Macbeth down. The Scots view their country as a patient whose only cure is blood that they will spill on the battleground.