Macbeth assumes that a forest will never come to get him, so he laughs off the third apparition the witches show him.
Macbeth misinterprets pretty much all of the apparitions that witches show him the second time, and it is his downfall. Of course, some are contradictory. For example, he is told to beware Macduff but also that no man born of woman can hurt him. Huh? Macbeth will find out later that Macduff was born by Caesarean-section and therefore can hurt Macbeth.
The third apparition is one of the most important ones. It is a child crowned with a tree in its hand. It does not take a genius to figure out that the child is Malcolm. Malcolm is very young, and was named Duncan’s heir. He is a danger to Macbeth, but Macbeth has been ignoring it. He assumes that since Malcolm ran he will not come back. He didn’t realize that Malcolm fled so he could come back with an army.
The third apparition warns Macbeth that the woods are going to come against him.
Be lion-mettled, proud; and take no care
Who chafes, who frets, or where conspirers are:
Macbeth shall never vanquish'd be until
Great Birnam wood to high Dunsinane hill
Shall come against him. (Act 4, Scene 1)
That is where the tree the child is holding comes in. When Malcolm attacks Macbeth, his men cut down branches to hide behind, disguising themselves as the Birnam wood. So when the apparition tells Macbeth that Brinam wood is going to come and get him, it is right!
Let every soldier hew him down a bough
And bear't before him: thereby shall we shadow
The numbers of our host and make discovery
Err in report of us. (Act 5, Scene 4)
Macbeth does not expect a forest to be moving. In fact, it really isn’t but it would look like it was once the army got closer. When they were far away, moving slowly, they would have blended right in with the woods.
The apparitions, like the prophecies, seem to help Macbeth at first. However, everything the witches show Macbeth leads him to his doom. The warnings of the apparitions are cryptic and leave Macbeth confused but confident, because he misinterprets them.