Why Is Birnam Wood Important in Macbeth?

This forest is celebrated in Shakespeare's Macbeth as the famous Birnam Wood. The prophecy of Shakespeare's three witches did come true, with the branches of trees from great Birnam Wood, nearly 1,000 years ago, camouflaging the advancing army against Macbeth.

How is Birnam Wood advanced?

Birnam Wood arrives at Dunsinane when Malcolm instructs his soldiers to hack off boughs from the trees of Birnam Wood to act as camouflage when they approach Macbeth's stronghold. Malcolm believes the branches will disguise the number of troops approaching Dunsinane, giving him a military advantage.

Which explanation best describes the significance of the Messenger's report that Birnam Wood is moving up Dunsinane Hill?

What does the messenger report he saw? He says he's spotted a grove of trees moving toward the castle. Basically, this means that Malcolm and his army are closing in on Dunsinane. It's also a fulfillment of the prophecy because Birnam Woods will be arriving soon at Dunsinane Castle.