When taken almost literally, it means someone is unintelligent or unequipped to talk to a specific person or about a subject. Although it has ironic connotations to mean that the
interlocutor or the person they are speaking with is
on another level/
wavelength.
Compared to my friend, big brain
Bojack, who is simultaneously discussing
Kant's Deontology and Quantum Mechanical
superposition, I have a small brain.