Some Pokemon types are simple. Their definitions are solid, and are unchangeable. For example, Fire Pokemon. Fire is the type of not just fire, but magma/lava and most of all, heat. Or Ice, being the type of frozen water in the form of glaciers, icicles, and snow.
Others... not so much. Either their type energy isn’t fully explained how it works or where it comes from, or it’s explained, but difficult to explain. As an example of the the first, we have the Dragon Type. Dragon energy is not fully explained, or understood, due to how volatile it is (which makes it so hard to contain and do research on), but it is thought to have been first used and harnessed by the first Dragon, or the common ancestor of the True Dragons: Tyrantrum, the Despot Pokemon, living roughly 100 million years ago during the Cretaceous Period. This Pokemon would go on to split off into hundreds of different branches all across the world, creating the Dragon typing as we know it (save for those few species that haphazardly stumbled across it in a case of convergent evolution, such as Exeggutor).
As an example of an explained but difficult to explain type is Ghost and Dark, which are closely linked by an strange matter known as Distortion. Distortion is a corrosive substance, though its less corrosive on solid objects and more corrosive upon time and space as a whole. Most ghosts naturally hail from the Distortion World, or from some other world closely related to it, as they are almost entirely compromised of Distortion matter (those ghosts who are not from the Distortion World spawn as a result of a recent death and a recently deceased soul being exposed to distortion matter, and becoming a ghost Pokemon as a result). They are not fully corporeal, able to shift in and out of existence as they please, and gleefully break the laws of reality as if a game. Dark Types, on the other hand, do not hail from the Distortion World, and are corporeal, yet they wield distortion as a water type does with water. Their bodies naturally produce the substance (often at the cost of their own health and survivability), and as a result, the distortion is far more concentrated and potent, which overloads and disrupts Ghost Pokemon.
However, by far the hardest type to explain is Fairy. Ever since its discovery and classification in Kalos roughly 200 years ago, Fairy type energy has defied full explanation or understanding, and the creatures that wield it are hard to fully classify as “alive.” At this time, it is believed that the first fairies, like Ghosts, hailed from another plane of existence. However, unlike the ghosts, Fairies accepted a degree of physicality to become somewhere in-between corporeal and incorporeal as whimsical and magical, somewhat grounded in reality.
The most common theory, though, is that parts of them are and are not corporeal. Take the Gardevoir/Gallade lines, for example. It’s been found that the only truly solid part of their body is their signature chest spikes. Instead, the rest of their anatomy is somewhat incorporeal, requiring no organs or muscles, simply being controlled by the will of the Pokemon (Fairy types have been found to have a pseudo-nervous system made of pure, concentrated energy running throughout all of their limbs and appendages, which is likely how they manipulate their bodies). Since this spike is the only truly corporeal part about them, it is the only thing that does not dissipate after death, as the rest of the body simply fades away while only the spike remains.
Fairy Types continue to defy explanation, and even the most telepathically adept fairies do not or cannot provide concrete explanations, so likely, the Fairy type will remain a mystery for years, if not decades to come.