Everything we know about the world comes to us through our five senses, which we experience internally (as neurons firing, though Descartes wouldn't have put it that way). How do we know those firing neurons correspond to anything real out in the world?
After all, if our senses were being systematically and ubiquitously deceived, whether by demon or daemon, we would have no way of knowing. How would we? We have no tools other than our senses with which to fact-check our senses.
Because we can't rule out the possibility of such deception, we can't know for certain that our world is the real world. We could all just be suckers.
This kind of skepticism sent Descartes on an internal journey, searching for something he could know with absolute confidence, something that could serve as a foundation upon which to build a true philosophy. He ended up with cogito, ergo sum — "I think, therefore I am" — but that has not fared well with subsequent philosophers.
Start your Friday off right and get those meaning-of-existence wheels spinning in your brain!
(Bonus comment: imagine trying to explain this concept to Donald Trump? Better yet, imagine Donald Trump trying to explain this concept to someone else?)