Plot in Macbeth
Despite Macbeth's victory against Macdonwald, the Sergeant tells Duncan that the Norwegian (Norweyan) lord renewed his attacks against the Scottish armies with more men and more weapons making defeat seem like a possibility.
Macbeth is interested and perplexed with the information the witches have presented him with. He attempts to rationalize their information with what he knows: the Thane is Cawdor is loyal to the King, and there is no prospect for him to be King of Scotland. However, he doesn't yet know about the Thane of Cawdor and demands that the witches tell him how they know such things.
Banquo points out that Macbeth is enraptured by the words of the witches. They have greeted Macbeth with his current title, Thane of Glamis, and predicted his title of noble having, Thane of Cawdor, and even of royal hope, King of Scotland. Their predictions have a strong effect on Macbeth and how he reacts to soon learning that he has in fact become Thane of Cawdor.
Duncan has named his son Malcolm as the Prince of Cumberland. This makes Malcolm next in line to the throne after Duncan, which is important because in medieval Scotland, the firstborn son did not automatically become heir to the throne. Notice how Macbeth reacts to this news in an aside to himself momentarily.
Duncan has two sons, Malcolm and Donalbain. This is the first mention of Donalbain in the story. If Macbeth were to ensure his claim to the throne, he would have needed to kill both of Duncan's sons as well.
As Macbeth leaves the courtyard to carry out the deed, Lady Macbeth enters it. Since there is no change of scene, her brief soliloquy is meant to pass the time until Macbeth returns from Duncan's chamber.
In other words, the reason that tells us to wait for a moment and not act too hastily. Macbeth says his love for Duncan was so strong that he killed the grooms in a moment of passion. Consider how this explanation of why he killed the grooms compares to Macduff's nonviolent reaction to seeing Duncan's body.
Presumably, the Porter has made his way to the gate and decided to stop playing his game of imagining himself as a porter of the gate to hell.
As Macbeth and his wife leave the courtyard, the scene doesn't change. The porter, slowly wakened by the knocking from his drunken sleep, makes his way to the door. His speech helps move the time along as Macbeth cleans his hands and tries to pretend that he was asleep while Duncan was murdered.
Recall how earlier in the play, before killing Duncan, Macbeth states that he knows he'll pay for the murder of Duncan with his own soul. Here, he acknowledges how his soul ("mine eternal jewel") has been given to the devil ("the common enemy of man") in order to fulfill his ambitions, and how he still intends to kill other who threaten him.
Considering what happens later in this scene, this question is asked under false pretenses. Macbeth pretends to inquire as a friend, but his ulterior motive is to inform himself of Banquo's plan, so that he may know Banquo's whereabouts to set the ambush.
Banquo earlier declared that he stood opposed to all signs of treason; however, in this short soliloquy, he appears to have a different attitude. Even though he suspects Macbeth of murdering Duncan, he makes no effort for vengeance. Instead, he contemplates the prophecy that his descendants shall be kings, hoping that this prophecy shall also come true. While he isn't trying to make this happen, he is serving Macbeth with the hope that his house will benefit.
The reasons for why Macbeth tells his wife to heavily praise and favor Banquo are unclear. He has not told her about his plans to have Banquo murdered, so these words are possibly designed to deceive her. Another reason, although less likely, is that he knows there is a chance the plot would fail, and he wants her to act naturally.
The Second Murderer tells the First that he shouldn't be suspicious of the Third Murderer. He says that the newcomer has followed the same instructions the first two received from Macbeth about the time, place, and nature of the crime, exactly as they have--as shown by his choice of words, "to the direction just."
As short as it is, this scene is the climax, or turning-point, in the drama. Macbeth has been fortunate thus far with his plans, and by all appearances he has had a successful career without any setbacks. Fleance's escape is Macbeth's first sign of bad luck. Notice how after this moment, other events will contribute to Macbeth's downfall until the end.
This statement represents a type of formulaic response to the king's toast, in which the Lords acknowledge their host and king and drink to the toast he has proposed.
The ghost has momentarily left the room, but Macbeth continues his rant. In this line, he recalls the information the First Murderer told him about the "twenty trenched gashes" on Banquo's head as he goes on about blood, death, and the rising of the dead. Readers should remember that the assembled Lords would be watching and listening to most of what Macbeth and his wife are saying and doing.
On the stage, before saying this line Lady Macbeth would have moved across the room to her husband and drawn him aside. This question is an attempt to shame him back into the proper frame of mind by accusing him of cowardice.
Hecate chastises the other witches for their attention to Macbeth. She states here that he "loves for his own ends," which means that Macbeth only listens to the witches for his own purposes and not for any kind of loyalty or love. This concept is another example of how dissimilar this scene is with the other scene with the witches, because Hecate is referring to attitudes and feelings that Shakespeare did not discuss nor mention early in the play.
Notice how in this speech Lennox asks several questions about the main events in the play. These are means to be rhetorical and ironic, as having witnessed Macbeth's strange behavior earlier and the poor condition the country is in, Lennox now believes Macbeth is guilty of murder and that he needs to be overthrown.
Lennox states that because Duncan had been killed, Macbeth's pity was not able to help Duncan. However, what Lennox is implying is that Macbeth did not feel sad or have sympathy for the king until after he had killed him.
This scene serves as a counterpart to Act II Scene iv by having relatively minor characters converse about the current state of affairs in Scotland and public attitude towards Macbeth. Whereas in the earlier scene, public opinion mourned the loss of Duncan and did not suspect Macbeth, here we see how the nobility has recognized Macbeth as a tyrant and efforts are being made to raise an army to overthrow him.
To hew means to cut something (typically wood or a tree) with an axe. With these orders to create bows to conceal the numbers of the army, Malcolm is actually fulfilling part of the witches' prophesy: That the Wood of Burnam would move against Macbeth.