At the end of Act 5, sc.4 of Macbeth, Malcolm gives the following order to his army:
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.
Shakespeare could not represent an entire army on his stage, so he resorts to his usual technique of having a character describe what is happening offstage and out of sight. In Act 5, sc. 5, a Messenger reports to him as follows:
As I did stand my watch upon the hill,
I looked toward Birnam, and anon, methought
The wood began to move.
From the strong emotion displayed by the Messenger and the violent reaction of Macbeth to the news he brings, the viewer can imagine the effect produced by an entire army marching toward the castle with each man holding a branch of a tree above his head. The Messenger describes it as "a moving grove." Macbeth appears totally unnerved by this development, not so much because it is an advancing army as because it corresponds exactly to what one of the witches' Apparitions foretold in Act 4, sc.1:
Macbeth shall never vanquished be until
Great Birnam Wood to high Dunsinane Hill
Shall come against him.