I think "slots" is a poor choice of terminology. It makes it sound like an abstract container you fill with a spell to use later, when in fact it's the complete opposite.On the topic of the Magic system, I'm finding that new players are having difficulty parsing the "prepped spells" vs "spell slots". It hard to explain to someone totally new to this kinda thing.
also, MM is auto-hit 3d4+3 that can be spread out if necessary...
Sadly, I'm not sure the English language has a concise, precise word or phrase for the thing a spell slot actually represents. It's not exactly a magic point system; it's more like an "arcane ammo" system, but that sounds dumb.
Maybe they could have referred to them as "spell uses": You get so many spell uses per day. You expend a 3rd level spell use to cast fireball. You can expend a higher-level spell use to increase the number of missiles from a magic missile spell. Too late to change it, though.
Anyway, the best way to explain it is thus:
Spellbook: All the spells you have access to
Prepared spells: All the spells you can fit in your head at one time
Spell slots: The amount of arcane energy you can expend in one day, using any spells you have prepared. (Or just call them spell uses instead of slots!)