The "imperial theme" is the theme of the three greetings enunciated by the witches. Their first two greetings are solely intended to lead up to the final greeting of "All hail, Macbeth, that shalt be king hereafter." The first two greetings are true.
The First Witch says:
All hail, Macbeth! Hail to thee, Thane of Glamis!
The Second Witch says:
All hail, Macbeth! Hail to thee, Thane of Cawdor!
This has come true, but Macbeth is not yet aware of it. It is more effective that he should not be aware of it at that moment because it makes the witches seem to have psychic powers.
The Third Witch says:
All hail, Macbeth, that shalt be king hereafter!
It is not until after the witches vanish and Macbeth and Banquo encounter Ross and Angus that Macbeth learns he has been appointed Thane of Cawdor by King Duncan as a reward for his great military services. Evidently the title of Thane of Cawdor is more prestigious than that of Thane of Glamis, so the two first greetings can be said to be part of the "swelling act" culminating in the ultimate title of King of Scotland. The three greetings all constitute the "imperial theme" because they are intended to lead up to the prediction that Macbeth will hold the imperial power of King. He says to himself:
Two truths are told,
As happy prologues to the swelling act
Of the imperial theme. (Act I, Scene 3)
Since this play is called The Tragedy of Macbeth, Shakespeare tried to preserve some audience sympathy for his protagonist in spite of Macbeth's truly foul deed in murdering his monarch. The witches seem intended to ameliorate a portion of Macbeth's guilt by making it seem foreordained that he would become king and would have to commit regicide in order to accomplish it. If the witches are deceiving him, that only helps to dilute his guilt. Macbeth's wife assumes a large portion of the guilt by insisting that her husband commit the murder and acting as his accomplice. Duncan himself tells Macbeth:
More is thy due than more than all can pay. (Act I, Scene 4)
In other words, if Macbeth took the whole kingdom he would only be taking a part of what he was entitled to. And Fate seems to have arranged it so that Duncan and his two sons, Malcolm and Donalbain, will be staying overnight in Macbeth's castle for the first time ever.
Shakespeare even shows Macbeth torn with doubts, fears, and aversion to commit such a dastardly deed. It seems evident that he initially planned to kill Malcolm and Donalbain along with their father but was so unnerved by killing Duncan that he couldn't shed any more blood. He is subject to hallucinations. He imagined he heard a loud voice calling "Sleep no more!" He had to flee to his chamber, and he wouldn't even go back to smear the sleeping grooms with blood from their own daggers which he had forgotten to leave at the scene of his crime. True, he is a murderer, but he is an extremely reluctant murderer—if that is any excuse.
Ah, good question. The two truths are the two of the witches' predictions that have already come true, leading Macbeth to believe that a third will come true: he will be king. The "imperial theme" is the idea that he'll be king.