In Act 5, Scene 4, Malcolm issues orders to have each soldier cut down a bough from the trees in Birnam Wood and hold it before him; in this way, Macbeth's lookouts will not be able to get an accurate count of his men. (This is also the way the Weird Sisters' prophecy about Birnam Wood coming to Dunsinane Hill will come true.) Malcolm plans to advance on Dunsinane because, as Siward informs him, Macbeth keeps to the castle now; Macbeth will not risk himself by coming out to fight. At this point, Malcolm says, "Both more and less have given him the revolt, / And none serve with him but constrained things / Whose hearts are absent too" (5.4.16-18). Both nobles and commoners alike have revolted against Macbeth's rule, and the only people who still serve him are those who do so for duty's sake, but even their hearts are not in the fighting. Macbeth, then, has no real choice but to hole up in his fortress and hope it will withstand Malcolm's army's attack. This is why Malcolm plans to advance; in order to destroy Macbeth, he really has no other choice but to take Dunsinane.
In Act 5, scene 4, of Shakespeare's Macbeth, Malcolm (Duncan's oldest son, named to be the dead king's successor) has decided to attack Macbeth at Inverness. Upon defeating the tyrant who murdered his father, Malcolm plans to take back the throne and rid Scotland of its murderous king. In order to accomplish this, Malcolm gives strategic instructions to his men.
As they approach the castle, Malcolm and his army come to a wooded area that Menteith identifies as Birnam Wood. In order to camouflage their numbers and surprise Macbeth (covering the sizable force moving to engage him), Malcolm tells his soldiers to cut branches from the trees and use them to shield themselves. (It is in this way that Birnam Wood appears to move.)
Let every soldier hew him down a bough,
And bear't before him: thereby shall we shadow
The numbers of our host, and make discovery
Err in report of us. (V.iv.6-9)
Malcolm's plan is to show Macbeth's lookouts a false front so they will be unable to ascertain the true number of soldiers preparing to attack. The lookouts will provide Macbeth with a number smaller than the actual army advancing on Inverness. By the time Macbeth realizes the mistake, it will be too late for him to defend against such vast numbers, especially because (as Malcolm reports) aristocrats and commoners (everyone with a heart) have fled, abandoning Macbeth to his fate. (15-18)
Malcolm's plan is to camouflage themselves with tree branches from Birnam Wood, so that Macbeth won't notice them until it's too late. This tactic, of course, will fulfill one of the prophecies of the witches that Macbeth will have nothing to fear until Birnam Wood begins to move.
In Act 5, scene 4, Malcolm knows that Macbeth's side is weakened. He had said earlier that Macbeth was "...ripe for shaking" and he was now ready to capture the city. Just like a ripe fruit, he will shake Macbeth to make him fall. He comes up with a military strategy whose main aim is not to hide the advancing army but to confuse Macbeth about the number of the soldiers. He orders the soldiers to "...hew...down a bough" to carry with them so that the rivals would be confused. Macbeth had planned to defend the fortified castle and Malcolm knew this. However, Malcolm's plan would work because Macbeth's army will not know the numbers they were up against. This was a brilliant military strategy.
Malcom's miliatry plan was to make all his soldiers break off a branch from the trees in Birnam wood and hold it in front of themselves so as to conceal their appearance, while moving towards Macbth's castle in Dunsinane. By doing this, he felt, it would create confusion in the minds of Macbeth's soldiers, thereby giving them miscalculcated and wrong reports regarding the strength of the opponent's army. Hence, leaving them unprepared. :)