While anticipating Banquo's murder, which he has already ordered (along with Fleance's), Macbeth is obviously disturbed and his wife asks what's bothering him. By her logic, since he cannot undo what he has done, he should just forget about it. He responds that "We have scorched the snake, not killed it. / She’ll close and be herself whilst our poor malice /
Remains in danger of her former tooth."
Just what is this "snake" he fears? He does not say, but it could be anyone who is aware of his regicide. It could also be that he's thinking of Malcolm and Donalbain, who are the rightful heirs to the throne but have escaped his grasp for the time being. If Macbeth is to retain his crown, they must be dealt with. Essentially, though, Macbeth is aware soon enough, people will talk and conspire against him, having put two and two together.
"But," he adds:
Let the frame of things disjoint, both the worlds suffer,
Ere we will eat our meal in fear, and sleep
In the affliction of these terrible dreams
That shake us nightly.
In other words, I'd rather heaven and hell themselves fell apart than we had to live in constant fear and deal with these terrible dreams. He is struggling to justify the murders he has commissioned.
Duncan is in his grave.
After life’s fitful fever he sleeps well.
Treason has done his worst; nor steel nor poison,
Malice domestic, foreign levy, nothing
Can touch him further.
Here, he voices some envy of Duncan. Duncan may be dead, but "he sleeps well," beyond responsibility, the fears, and cares of the world.
Macbeth is basically a good man gone bad, so he struggles with his choices. His conscience plagues him, giving him nightmares and visions from the beginning of his treason. This is just another manifestation of that. He best describes his torment a bit later in this same conversation: "O, full of scorpions is my mind, dear wife!"