What is Macbeth about, in simple terms?
Macbeth is the story of an ambitious man who takes some bad advice. At the beginning of the play we meet three witches who will become important in the story because they provide prophecies that Macbeth takes to heart. Macbeth is a noble (a thane), but he is not particularly important. He is just an ordinary soldier and by all accounts brave and honorable. The witches tell him that he will be Thane of Cawdor (a higher title than he has), and that he will be kind. They also make another interesting prophecy about his friend and ally Banquo, that his sons will be king.
Macbeth is interested in these prophecies, but Banquo is suspicious. When Banquo and Macbeth meet with the king, King Duncan names his son Malcolm heir to the throne instead. This upsets Macbeth, even though there is no reason for him to be named king. He makes a startling proclamation, in an aside, which basically means he is talking to the audience (or himself).
The Prince of Cumberland! that is a step
On which I must fall down, or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires (Act 1, Scene 4)
In this speech, he reveals his ambition. He shows that he wants to be king, and he is willing to take action to become king.
When Macbeth returns home, we meet his wife, Lady Macbeth, and realize she may be even more ambitious than he is. He told her in a letter about the prophecies. She wants him to kill Duncan when he comes to Inverness, the castle where the Macbeths reside. Macbeth balks at the thought, and she accuses him of being less than manly. This woman really wants to be queen! Macbeth follows her carefully laid out plan, despite some misgivings, and they frame Duncan’s guards. Both Macbeths are really good actors and pretend to be shocked when Malcolm and another thane, Macduff, find the body.
So all is proceeding according to plan, right? No. Strange things are happening in the kingdom, spooky and paranormal things. Remember Banquo? He heard the prophecies too, and he is pretty suspicious.
Thou hast it now: king, Cawdor, Glamis, all,
As the weird women promised, and, I fear,
Thou play'dst most foully for't: yet it was said
It should not stand in thy posterity,
But that myself should be the root and father
Of many kings. (Act 2, Scene 1)
Basically, in this speech, Banquo is saying that Macbeth has the crown (he is talking to himself, and when he says “thou hast it now,” he is speaking metaphorically to Macbeth and he means the kingship, and the prophecies). He is worried about how Macbeth got where he is. He suspects murder. Macbeth knows he suspects, or at least suspects suspicion, and decides to have Banquo and his son killed. What’s another murder? Killing Banquo and his son will avoid the suspicion problem and also take care of that teensy problem of the third prophecy, where it says Banquo’s sons will be kings.
Macbeth does not stop there. Macduff is no slouch either. Malcolm and Donalbain, the king’s sons, both left. Macbeth would like to kill as many as he can, and he sends murderers to kill Macduff and they succeed in killing his wife and son. Malcolm rallies Macduff, after determining that he is loyal, and decides that enough is enough. He will gather soldiers and go to fight Macbeth.
Macbeth has reason to fear Macduff. The witches are not done making prophecies! They sent Macbeth a few more, and the ones concerning Maduff were confusing. He was told in no certain terms to be worried about Macduff.
Macbeth! Macbeth! Macbeth! beware Macduff;
Beware the thane of Fife. Dismiss me. Enough. (Act 4, Scene 1)
That seems to contract this other prophecy.
Be bloody, bold, and resolute; laugh to scorn
The power of man, for none of woman born
Shall harm Macbeth. (Act 4, Scene 1)
How can Macduff harm Macbeth if he was born of woman? Wasn’t everyone? Also confusing is a prophecy that seems to show a long line of Banquo’s heirs. Macbeth thought he killed Banquo’s son. Whoops.
Macbeth is saddened to learn, as he prepares for battle, that his wife has committed suicide. She goes mad at the thought of her part in Duncan’s death, unable to get the blood off of her hands metaphorically; she still sees it in her mind’s eye. Macbeth ponders the meaning of life in a beautiful speech that shows that he is not just a callow murderer, but actually has some humanity.
Life's but a walking shadow, a poor player
That struts and frets his hour upon the stage
And then is heard no more: it is a tale
Told by an idiot, full of sound and fury,
Signifying nothing. (Act 5, Scene 5)
However, this speech also basically means that Macbeth is suicidal. He is aware that his days are numbered. Malcolm and Macduff’s army is descending on him soon quickly. He is losing faith in his confusing prophecies and his own battle prowess. In the end, it turns out that Macduff was “from his mother's womb/ Untimely ripp'd” (Act 5, Scene 8) and therefore by technicality not “woman born.” Macbeth loses all confidence in fighting upon hearing this. Macduff is able to kill him, and he does. He beheads Macbeth. Malcolm ends the war, and takes over the kingdom.
I hope that helps. I have included some links you might find useful too.