Answer:
Macbeth is contemplating the murder of Duncan
Step-by-step explanation:
In this scene, Macbeth is shocked that the weird sisters' prophecy has come true: he is now Thane of Cawdor. Since they also said he would be king, he is now mentally combatting how that part of the prophecy will occur. He could just wait it out and let it happen without him taking any action - without "his stir" - but he also contemplates killing Duncan to gain the throne. "That suggestion" is Duncan's murder, emphasized when he later states, "whose murder yet is but fantastical." Macbeth hasn't acted yet; he's just thinking about it, and it scares him.