In Act 4, Scene 1 ofMacbeth, all three apparitions give information about Macbeth's future.
The first apparition is an armed head who tells Macbeth to beware Macduff. The second apparition is a bloody child tells him that none of woman born shall harm Macbeth. The third apparition is also a child, crowned, with a tree in its hand. The child tells Macbeth that he will not be vanquished until the Great Birnam Wood comes against him. The fourth apparition is a line of 8 kings, with Banquo last, holding a mirror.
It is debatable whether or not the witches are real. Even in the case that they are not real, they represent potential (and in Macbeth's case, the potential for evil.) Since their existence is uncertain, since they are seemingly unreal, their suggestions to Macbeth throughout the play represent unreality becoming reality. The same can be said for the apparitions. They are ghosts, projections of light, mass-less, seemingly unreal. And yet, they become real. The portents of the apparitions are also seemingly unreal or so unbelievable that they could not be true. Macbeth feels he can deal with Macduff without a problem. Macbeth can't imagine a man who is not born from a woman and he can't imagine a forest rising up as if it were alive. And Macbeth reasons that if Banquo is dead, he could not be king.
The four apparitions all seem impossible, or unreal, to Macbeth. Therefore, he won't believe these seemingly unreal, unbelievable premonitions can come to pass.