William Shakespeare's Macbeth is one of his tragic plays. Macbeth, the tragic hero, is lead to his demise by his ambitious nature. In act one (scene three), Macbeth has been told by the three witches that he will be both Cawdor and King. Not knowing that King Duncan had already named him the new Thane of Cawdor, once he hears the news from Angus and Ross that he is now Cawdor, Macbeth begins to believe in the prophecy. 

His first aside is found in lines 124 and 125 (I, iii). He states:

[Aside.] "Glamis, and Thane of Cawdor! The greatest is behind. 

This aside, a comment made to the audience or another character (and not all of the characters on stage), illustrates that he recognizes the truth behind the prophecy of the witches. He was already Glamis; he just received Cawdor, and king is the only title left (meaning that two the the three have been possessed). The "greatest behind" refers to the fact that he already has the majority of the titles referred.