In this short scene, Malcolm instructs his troops to disguise themselves by cutting down the boughs of trees from Birnam Woods and carrying them in front of them, so the enemy (Macbeth) can't see clearly how many men are approaching. One literary device Shakespeare uses here is dramatic irony. Dramatic irony occurs when the audience knows what the people in the play don't. We have left Macbeth in the previous act clinging emotionally to the witches' prophecy that says he can't lose unless Birnam Woods move--and we, as an audience, now know what Macbeth doesn't--that the woods will "move" because the soldiers will cut down the boughs and carry them forward. We can at this point guess, if we haven't already, that the battle is not going to go well for Macbeth.
The scene also uses characterization, characterizing Macbeth's abilities as a king. We learn that Macbeth wants a siege so that his men can't desert him, which they have been doing. This shows he is unable to command loyalty; he is not a good ruler. This not only characterizes Macbeth but foreshadows his doom: even if he were to win the battle, his leadership abilities point to his eventual defeat.
At the end of the scene, Shakespeare uses the literary device of verse, with the rhymes "know/owe" and "relate/arbitrate." The rhyming verse emphasizes the importance of what is being said:
The time approaches
That will with due decision make us know
What we shall say we have and what we owe.
Thoughts speculative their unsure hopes relate,
But certain issue strokes must arbitrate.
Towards which, advance the war.
In other words, we can talk, speculate and hope, but, in the larger picture, it's action that will make the difference, the action of war. In other words, we are now in suspense, anxious for the resolution to come as the two armies meet.