In Shakespeare's Macbeth, Macbeth has been complacent about the safety afforded him by the witches' predictions in Act Four. This is the second set of predictions they have delivered to him, but this time the witches are under orders from Hecate, Queen (or "Goddess") of the Witches, to lead Macbeth to his doom. She says this is easy to do with mortals by providing them with a "false sense of security." Macbeth comes away with a false security based on the information from the witches—information Macbeth demands of them—as if he is in control of these evil creatures. Macbeth's ambition has truly blinded him, and he behaves foolishly.
With the new predictions, the witches have given Macbeth information that is misleading in that things are not as they seem. Macbeth takes the prophecies literally, while the witches have spoken the predictions figuratively. We see this as Malcolm's army gathers to attack; they cut down tree branches to camouflage themselves because they do not want Macbeth's scouts to know how many of them there are—a strategy they hope will give them an advantage when they fight Macbeth's army.
When the soldiers advance, Macbeth's scout reports that it looks like the woods are moving. The original prediction was:
Macbeth shall never vanquish'd be until
Great Birnam wood to high Dunsinane hillShall come against him. (IV.i.103-105)
Macbeth has assumed that the woods cannot move—it's impossible. When he hears the report from his soldier, he is incredulous and furious. The witches have misled Macbeth because it looks as if the woods are moving: it his then that Macbeth realizes that he has been tricked.
The men referred to in the question are Malcolm's forces which are moving against Macbeth's castle at Dunsinane. In 5.4 Malcolm's army arrives at "the wood of Birnam" (5) which is near the castle. Malcolm gives the following order: "Let every soldier hew him down a bough / And bear 't before him. Thereby shall we shadow / The numbers of our host . . ." (6-8). The idea is that from the castle it appears as if the wood itself is moving against Macbeth's position. This is important because is shows how one of the prophecies of the witches (see 4.1) comes true. Shakespeare gives Macbeth's reaction to this news as well as his repetition of the witches' prophecy in the final speech of the next scene. Particularly important is that he states that he begins "To doubt th' equivocation of the fiend, / That lies like truth" (5.5.49-50).