What does "proceeding from the heat-oppressed brain" mean?
In act two, scene one, Macbeth begins to hallucinate and sees the image of a bloody dagger leading him to King Duncan's chamber. Macbeth immediately questions the "fatal vision" and wonders if it is simply a figment of his imagination. He questions whether the image of a bloody dagger is a "false creation," which is "Proceeding from the heat-oppressèd brain." Macbeth is wondering if he is physically suffering from a fever (this is the meaning of the phrase "heat-oppressed brain"). Macbeth is concerned that his intense emotions may have caused his blood to become overheated, leading to altered perception. Macbeth is experiencing a myriad of emotions, including doubt, anxiety, stress, and pressure. He is essentially wondering if his heated emotions have affected his brain, causing him to hallucinate and see an imaginary bloody dagger. At the end of the soliloquy, Macbeth acknowledges that his emotions, thoughts, and tortured soul (rather than some external element) are responsible for the "fatal vision," and he proceeds to enter the king's chamber to commit regicide.
No doubt this is an incorrect self-diagnosis of his mental condition based on a faulty understanding of human physiology and psychology. Macbeth is apparently thinking that his turbulent emotions have caused his blood to become unnaturally heated and the warm blood entering his brain might be causing him to have hallucinations, in one of which he sees a floating dagger leading him toward Duncan's chamber. He is not completely sure whether the dagger is real or imaginary. This is the first of many hallucinations Macbeth will be experiencing once he has committed himself to murdering the King. The dagger Macbeth thinks he sees may have been "real" in the sense that Shakespeare may have actually arranged to have a dagger suspended from a dark string dangling in front of the actor playing Macbeth and apparently leading him on to commit his treasonous crime.