In Act Two, scene one, Shakespeare's tyrannical usurper in Macbeth, hears a bell. It is the central focal point for the commencement of Macbeth's plan to murder Duncan: his King, his cousin and his friend. He has said:
I go, and it is done: the bell invites me. (70)
The best way to understand the meaning of the line is to view it in context with the rest of the speech, even the rest of the play. It is a pivotal moment. This simply means that the plot development to follow hinges upon what happens in this time of decision for Macbeth.
Background: Macbeth is a valiant soldier, well-respected by his peers, and greatly loved by his King. However, Macbeth's flaw—his tragic flaw which he admits to...that will ultimately lead him to his downfall—is what he calls his "vaulting ambition." He is so ambitious that his desire to achieve almost trips over itself in trying to get more—more money, more power. It's not that he is poor; Duncan has just given him the title, lands and wealth from the traitor Cawdor. However, Macbeth cannot control his consuming desire to have and be more—like an addiction to a drug, he cannot control it: it controls him.
Macbeth meets three witches on the heath after the battle with Norway, and they say that he will be Thane of Cawdor and king one day. When Ross and Angus find Banquo and Macbeth, the King's reward of what Cawdor has lost is bestowed on Macbeth. Realizing the witches' prediction that he would be Thane of Cawdor has come true, he believes the rest of their predictions.
All hail, Macbeth, that shalt be King hereafter! (I.iii.53)
Macbeth cannot resist the possibility. As he thinks about it, the idea planted by servants of evil, he cannot help but believe he will be king.
[Aside.] Glamis, and Thane of Cawdor!
The greatest is behind. (124-125)
Macbeth begins to think about becoming king—what is left to come. Lady Macbeth is ready when he comes home, to talk him into a plan to kill Duncan, so she can be queen. He thinks about it, but hesitates for he loves Duncan, and his King has been good to him.
Lady Macbeth has prepared herself that Macbeth may be too weak ("too full o' the milk of human kindness...") to commit murder. So she begins to insult his manhood, call him a coward and nag him to death. He defends himself and gets angry, but his wife continues to work at him until he comes to the point where he must make a decision. We know he has decided when he says:
If we should fail? (I.vi.66)
It is the last hurdle he needs to overcome in order to agree. Once he asks this question, it is clear he is committed to the course of action necessary—as long as they don't get caught. Lady Macbeth tells him to be brave and no one will be able to connect the murder to them. And so he agrees:
I am settled, and bend up
Each corporal agent to this terrible feat.
Away, and mock the time with fairest show:
False face must hide what the false heart doth know. (90-93)
Macbeth is ready. He must pretend all is well; he cannot show on his face what he is thinking. He and his wife devise a plan; when she rings the bell, it is time for him to act. Prior to the bell, he hallucinates (or does he??!) a dagger hovering in the air, leading him on to Duncan's murder.
At one time unsure about this step, he now embraces it. The quote reflects that the signal is all that separates him from the act of murder.