In act 4, scene 1, the witches conjure apparitions, who actually foretell for Macbeth the conditions his downfall. Macbeth, however, in his megalomania and pride, takes this to establish his own invincibility. As the play approaches its conclusion, Macbeth will come face-to-face with the truth of these warnings as he faces his own defeat by Malcolm and Macduff.

The first apparition warns Macbeth to be leery of Macduff. From here, we come to the second apparition, which tells Macbeth,

Be bloody, bold and resolute: laugh to scorn
The power of man, for none of woman born
Shall harm Macbeth. (Act 4, scene 1)

Finally, the third apparition reveals that

Macbeth shall never vanquish'd be until
Great Birnam wood to high Dunsinane hill
Shall come against him. (Act 4, scene 1)

When reading these predictions, one can perhaps see why Macbeth interprets them in the manner that he does (especially given his lack of context as to the true meaning of these messages). After all, expecting a forest to start walking towards you would actually be extremely illogical (which is essentially what the third prediction states: that Birnham Wood will travel to Dunsinane). He treats the other of these two predictions in a similar manner, likewise referring to the impossibility of his defeat.

In reality, both these prophesies will be fulfilled in ways Macbeth himself has not expected. The prophesy about Birnham Wood is in reference to the advance of Malcolm's forces, who cut down branches from the trees before advancing against Macbeth, hoping to use the coverage to hide their numbers. This tactic gives the impression of a forest advancing towards him. Similarly, the other of these two prophesies is actually in reference to Macduff himself, who was delivered by C-section.