”Heat“ refers to the witches and Lady Macbeth, they are almost melting and reforming Macbeths brain just as plastic melts under heat.
Said in his soliloquy - shows his inner thoughts
“oppressed brain“ is oxymoronic. “brain” connotes free thinking and free will, yet “oppressed“ alludes to some higher power having the ability to control his brain (the witches)
This is said in Act 2: Scene 1 - so in just the span of around 1 act, he has become enveloped by the witches prophecies
Context: Freud and ID - he is a slave to his desires
