One or two sentences is not very much space to thoroughly summarize the entire play of Macbeth. Therefore, you are going to have to be very general in your summary. It should be easy to focus on the character of Macbeth, as the plot centers on him. You will also want to mention the themes of fate, ambition, and guilt—which are very central to the story's plot.
Keeping all of this in mind, your summary could go something like this: Macbeth, an ambitious Scottish general, hears the prophecy of three witches that he will become king. Therefore, at the urging of his wife, he murders King Duncan and assumes the throne—only to be consumed with guilt and suspicion before being vanquished by his enemies.