I'm inclined to think that there are, and perhaps must be, things that humans can't understand because of the limitations of our brains. Now, the term 'understand' might mean (i) 'understand at all (i.e., at least partly)' or (ii) 'understand completely'. On interpretation (ii), it's pretty clear that there are things we can't understand, because our finite brains can't grasp all of the infinitely many facts there are about even as ordinary a thing as my car. What's my car's exact mass right now? (No rounding allowed!) Even on interpretation (i), there may be things that humans can't understand.
The only organ with which we can hope to understand something is our brain. Why wouldn't our brain's finite capacity for information storage, calculation, and so on, be accompanied by a finite capacity for understanding things? Indeed, Colin McGinn has conjectured that some of the central problems of philosophy have endured for so long because, although they have solutions, our species lacks the ability to understand those solutions. See his Problems in Philosophy and this later paper. Jonathan Bennett sounds a similar theme in his article 'Descartes's Theory of Modality', Philosophical Review 103 (1994): 639-667, at 656.