What does a pine mean in Macbeth?

In Macbeth, the line "Till Birnam Wood remove to Dunsinane" refers to the witches' prophecy that Macbeth will only be defeated when Birnam Wood moves to Dunsinane Hill. This prophecy is fulfilled when Malcolm's army disguises themselves with branches from Birnam Wood, giving the appearance that the forest is moving toward Dunsinane and fulfilling the witches' prophecy.