In your own words? Well, you may need to figure that out for yourself :)
You can read the text of the play right here on eNotes, then you can take a look at the summaries, which will help you to understand the action. If your teacher asks for something "in your own words" there is no substitute for that.
In short, Macbeth is about a man quest for success, which he does it by underhand means by murder and killing people. He begins his reign racked with guilt and fear but later on, he becomes a tyrannical and totalitarian ruler, as he committed more murders to protect himself and save his throne. These bloodshed prove a breaking point as he become more cruel and outrageous to a point that it leads him to his death