The easiest structure one should follow when explaining the apparition scene and its significance in the play "Macbeth" should begin with where Macbeth began his downward spiral.
It is Macbeth's first meeting with the three witches that he learns of his climb to the throne. After telling Lady Macbeth of the prophecy, Macbeth realizes that he will not leave to fate the time-line in his obtaining of the crown. Instead, Macbeth decides to take matters into his own hands and take the crown now.
Once Macbeth murders for the crown, he begins to change. Things are not the same for Macbeth. Macbeth begins to hallucinate and is unable to sleep- given he has "murdered" his ability to sleep when he murders Duncan.
Unable to face the changes in his life, Macbeth goes back to see the witches. It is here where he sees the apparitions. The apparitions give Macbeth a false sense of security regarding the crown and Macbeth.
A full description of the apparitions and their impact on Macbeth can be found at the link below.
The end of the explanation should address the fact that Macbeth's false sense of security foreshadowed his ultimate demise.