What is the meaning of lines 122-127 of Act 1, Scene 3 of Macbeth? "Glamis, and Thane of Cawdor! The greatest is behind..."

In this scene, Macbeth and Banquo have been visited by the three witches (weird sisters). They have foretold (or suggested, depending upon your interpretation) that Macbeth is the Thane of Cawdor and will become king thereafter. They also say that Banquo will be less than Macbeth but greater. Clarifying this, they say that Banquo's sons will be kings. Macbeth asks them how he might become Thane of Cawdor but they vanish before answering him. 

Angus and Ross meet them and Ross tells Macbeth that he is to receive the title Thane of Cawdor. Macbeth and Banquo are shocked that this first suggestion by the witches has come true. Then Macbeth says to himself in an aside that he is now Thane of Glamis and Cawdor. He adds, "The greatest is behind." He means that the latter (the title "behind" the other) is the greater of the two titles; being Thane of Cawdor is better than being Thane of Glamis. 

Since the first suggestion has come true, Macbeth asks Banquo if he now has legitimate hope that his sons will become kings. Banquo answers that he is skeptical and asks to speak with Macbeth about it.