Macbeth was the king of Scotland at a time - the eleventh century - when Scotland's first language was mainly Gaelic. He was elected to the throne at a meeting in Scone after King Duncan was killed in battle, by Macbeth. Macbeth was related to Duncan on his mother's side. Duncan's wife fled Scotland with her sons Malcolm III and Donald after Duncan's death. There are various versions in the historical records, regarding Macbeth and even Duncan.
Shakespeare's Macbeth takes much of its account from the Holingshed Chronicles of England and Shakespeare in his Macbeth uses his poetic licence in the relaying of information. He was loyal to James I and keen to please his audience so may have extended his version of the truth to ensure he was sympathetic to the king's wishes.
The real Macbeth apparently reigned successfully for several years although some accounts label him a tyrant. He was defeated in battle during a successful attempt to return the crown to Duncan's heir Malcolm III.