Even Baldur's Gate 3's lead gameplay programmer had a hard time describing D&D's spell slots to their engineer, who has a PHD

pcgamer.com:

Dungeons & Dragons 5th edition, the tabletop ruleset which forms the bones of Baldur's Gate 3, is mostly straightforward. Gone are the days of adding double-digit bonuses to every roll—you roll one die, you add an attribute to it, you sometimes add proficiency bonus, and in rare cases you add that proficiency bonus twice.

But its annoying linguistic relic, spell slots, has plagued even Larian Studios CEO and Founder Swen Vincke, according to a recent interview with D&D's video producer and go-to interviewer Todd Kenreck. The subject comes up when they're discussing Vincke's dislike of bards—Kenreck asks Vincke what class he actually likes, and he answers «Wizards, always a wizard.»

This came with a caveat, though. «I hate the spell slots system, alright? I mean, my lead gameplay programmer had to explain to a really competent engineer—who has a PHD—what spell slots are. For an hour!» 

Spell slots are «a bit complex,» agrees Kenreck, doing an interview on Dungeons & Dragons' official channel. «When I was teaching my wife D&D it came up, and I was like, 'this is really hard to explain'.» 

Luckily, Baldur's Gate 3 is a video game, so its UI will often handle explaining things for you. If you can cast a spell, you can cast it. You never have to think about the rule implications behind the words 'spell slots'—but if you're hitting the pen and paper, then yeah, you need to fully understand how they work. Truthfully, I can see how even someone with a doctorate might be thrown through a loop.

You might hear the words spell slots and think, «ah, that is where I put my chosen spells», like equipment slots in other RPGs. This is a completely reasonable assumption, and it is also super wrong. Spell slots are more accurately