Malcolm gives orders for every soldier to carry one bough from a tree in Birnam Wood in order to make it impossible for Macbeth's forces to figure out how many men are coming against them. Malcolm's exact words are:
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. (5.4)
Act 5, Scene 4 ends with the stage directions "They exit marching." No doubt Shakespeare had about a dozen soldiers and officers, including Malcolm, Siward, Macduff, Siward's son, Menteith, Caithness, and Angus exit carrying leafy branches. It would, of course, have been impossible to show many more marching men carrying boughs from trees.The audience must imagine the effect of a huge army from the description given to Macbeth by the Messenger in Act 5, Scene 5:
As I did stand my watch upon the hill,
I looked toward Birnam, and anon methought
The wood began to move.
Macbeth has been assured by the Third Apparation evoked by the Weird Sisters in Act 4, Scene 1, that he has nothing to fear until
Great Birnam wood to high Dunsinane Hill
Shall come against him.
Macbeth is horrified and unnerved when he realizes that the supernatural assurances he has been relying on have misled him. He had been planning to keep his forces inside the castle and withstand a siege, but the news about the "moving grove" approaching makes him change his strategy and issue orders for his forces to meet the invaders outside. Evidently he thinks the approaching army is larger than it actually is, which would mean that his enemies could surround his castle and prolong a siege indefinitely.