[RPG] a spell slot in-lore, and how does it justify the limits on casting spells

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Spell slots are something that we, as players, expend when we want our spellcasting PCs to cast a spell. It is a resource to limit how many powerful spells we can cast in a day. But for our characters in-game, they don't exist. So what are they?

The best way I can think of to illustrate my question is via an example involving a Kenku.

Background

I was going to ask the question "Can a (non-spellcaster) Kenku cast a Verbal-component-only spell that they have heard a spellcaster cast via Mimicry?" I knew the answer would be that they can't, but I was wondering what the in-game justification is for this.

Out-of-game, the answer is that they do not have the Spellcasting (or Pact Magic) class feature, and therefore do not have spell slots to expend to cast the spell, but that just replaced one question with another; what is a spell slot in-game? Knowing the answer to this would justify why the Kenku perfectly mimicking the Verbal component of a spell doesn't work in-game.

Comparison

There are other terms we use: HP, AC, XP; these terms do not exist in-game. My PC won't know what "HP" is. HP has an in-game description, as explained further in this question: What does HP represent?

In short, from the PHB, pg. 196:

Hit points represent a combination of physical and mental durability, the will to live, and luck.

So those are things that my PC might know about and understand; luck, the will to live, etc. They make sense in-game and are something my character could talk about.

What I've Looked Up

A Wizard's Spellcasting class feature (PHB, pg. 114) only describes the mechanics of what a spell slot is to the player (I didn't check the Spellcasting class feature for all the other classes), and the Spell Slots section (PHB, pg. 201) simply says (regarding flavour):

Manipulating the fabric of magic and channeling its energy into even a simple spell is physically and mentally taxing, and higher-level spells are even more so.

But that doesn't explain why, say, a Kenku who has learned to mimic a Wizard's spell's Verbal components couldn't cast a spell (without being a spellcaster class themselves; i.e. they have no spell slots).

Sure, it might "tax" them, but surely they'd be able to pull it off at least once that day? Or is it so taxing to even say that specific word or phrase that they wouldn't actually be able to even finish saying it "without the proper training" (e.g. being a Wizard), and thus cannot "complete" the spell? (NB: This isn't my question, it's just included to show my train of thought.)

Question

So what are spell slots in-game? What in-game "thing" do they represent? Is there an in-game justification for why a character who has spell slots can cast a spell in-game, whereas a different character without spell slots could not (even if they can satisfy the spell's components; i.e. a Kenku perfectly mimicking the Verbal component)?

If the flavour of certain spellcasting classes would influence the answer such that all classes cannot be explained by one explanation (i.e. because Warlocks have Patrons, Sorcerers have "a spark of magic within them", divine casters have gods or ideals, etc) then this question can just focus on Wizards specifically and what their spell slots mean, since a Wizard's relationship with magic (i.e. "learning") is closer to how a Kenku "learns" the Verbal component via mimicry.

Also, I'm not particularly interested in a settings-specific answer, but if a specific setting would influence an answer, let's assume the Forgotten Realms (as it is the default setting of 5e).

Just to clarify: I don't think the Kenku should be able to do this (e.g. a level 1 Kenku overhears a high level Wizard cast wish, uh… no), my question is why not from an in-game/lore perspective.

Best Answer

As you noted, according to the PHB:

Regardless of how many spells a caster knows or prepares, he or she can cast only a limited number of spells before resting. Manipulating the fabric of magic and channeling its energy into even a simple spell is physically and mentally taxing....

So therefore, spell slots are an abstraction of arcane potential: how much magic a character can cast in a given day.

"Manipulating the fabric of magic" isn't just flavor; it's the abstract description of what is happening in-game.

There is therefore something, some source of arcane ability, that each caster has that allows them to manipulate the fabric of magic, something that non-casters do not have.

For different kinds of casters, that source of arcane ability is different. For some it is divine, or an otherworldly patron, or an innate magical gift, or the forces of nature.

You mentioned wizards specifically. Wizards have developed their arcane power after years of study:

From the PHB:

Though the casting of a typical spell requires merely the utterance of a few strange words, fleeting gestures, and sometimes a pinch or clump of exotic materials, these surface components barely hint at the expertise attained after years of apprenticeship and countless hours of study.

By definition, merely the verbal, somatic, and material components are insufficient, it is the mental expertise the wizard has gained over many years.

So, while the verbal, physical, and somatic components are (usually) a necessary component of casting a spell, they are not by themselves sufficient. There is something else, something internal, something attained in one way or another, perhaps by years of mental preparation, a divine or natural connection, an otherworldly patron, or some innate ability.

That's why merely mimicking the components isn't enough.

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