Macbeth:

Where are Menteith and the other noblemen going to meet up with the English soldiers to march against Macbeth? why is this important?

They will meet at the Birnam Woods.

This is important because one of the prophecies given to Macbeth is that he will be safe, he will not die until the birnam woods start moving towards him. ("Fear not till Birnam Wood do come to Dunsinane") He believes that this is impossible, and the witches did this to make him feel invencible and confident that he would never be defeated.

The strategy that Malcom proposes is that each soldier would grab a branch of the trees to hide the amount of people they actually are. When they are all behind the branches they start running towards the Dunsinane castle. Macbeth is informed about this, and his hope sinks when he is told by a messenger: "me thought, the wood began to move". Macbeth says "I pull in resolution and I doubt the equivocation of the fiend that lies like truth ( ...) " So his confidence fails and now he starts realizing the witches might have tricked him.