The reference to King Edward is in Act III, Scene VI. Another Lord is speaking to Lord Lennox:
Malcolm, Duncan’s son,
From whom Macbeth withholds his birthright,
Lives in the English court and is received
By the most pious King Edward with such grace
That the evil twist of fate takes nothing
From his high respect. There Macduff
Has gone to beg the holy king to send his aid
To support Northumberland and warlike Siward.
That, by the help of these two, together with Him above To ratify the work, we may again
Put meat on our tables, and sleep through our nights,Free from bloody knives at our feasts and banquets.We give “faithful” homage and receive free titles,Which we pine for now. And this report
Has so exasperated the king that he
Prepares for some attempt of war.
Lennox and the other lord are discussing what has happened to the kingdom of Scotland. Banquo has been murdered and his murder has been blamed on Fleance, who had to flee the country. Both lords suspect Macbeth, however. They believe he has killed both Duncan and Banquo. The lord tells Lennox that Macduff has gone to England, where Malcolm is now living. Malcolm is the true heir to the throne of Scotland, not Macbeth. Macduff hopes to plead with England’s King Edward for help in fighting against Macbeth. He plans to join up with Malcolm, come back with the English army, and defeat Macbeth.
This foreshadows the eventual overthrow of Macbeth. Macbeth has learned about the plots and is preparing for war. Lennox and the lord express their hope that Malcolm and Macduff will be successful in saving Scotland from Macbeth. Macduff does return with the English army to fight Macbeth. It is the English army that looks like Burnam Wood coming to the castle, as predicted by the witches in the beginning of the play.
Macduff is the one that eventually kills Macbeth - because he was "not born of woman." Macbeth thought he was invincible because the witches told him no one born of woman would be able to kill him.
You can read the text right here on enotes (see the link below).