https://tvtropes.org/pmwiki/pmwiki.php/Main/MaybeMagicMaybeMundane for an introduction to the concept.
Right now, many people (probably most) think that magic doesn't exist in our world. So there must be a perfectly mundane explanation for anything seemingly magical - it's quite probable though that you (or I, or even anyone) don't know it yet.
For a DnD player, the magic is very real. You've got your spellbooks etc. and you see occasionally the good old trusty Fireball spell. You know, a good old ball of fire appears on the mage's hand and flies to the impact point, everyone in 30 ft. radius rolls a Reflex save to get half instead of full Xd6 damage.
For a medieval person, or at least for a medieval person as we think he/she was (it's not like we can ask around), the question of magic was... less certain. A lot of things were quite mundanely explainable, some weren't.
Say, was the legendary Excalibur a magic sword, or was it by change an exceptionally well-made sword in an age when most swords were somewhat underwhelming? Was Achilles's armor magically impenetrable, or was it just a well-worked high-tin-content piece?
An interesting applied question is how can a GM, with judicious usage of concealed rolling, maintain uncertainty? Does this NPC weapon wield a weapon with magically enhanced damage, or is he just naturally rolling high damage? Is this pendant really cursed, or is the wearer plain unlucky with saving throw rolls? Does this armor piece have a magical enhancement, or do the enemies just roll too low to get past its regular DR?
It appears that there is quite a possibility for an enterprising GM to do just that - with some concealed rolling, of course, and a careful selection of spells involved. That is, the spell effect is involved from a purely mechanical Doylian perspective; however, from the Watsonian point of view, the observed effect cannot be distinguished from a stroke of luck.