This is an optional rule for wizards (and only wizards) in D&D, or other systems that do magic like D&D (vancian, nine spell levels, components, etc). It adds two additional ways to cast spells, but also changes the preparation time for spells. A spell now takes thirty minutes of time (doubled for each level above first) to prepare.
ON THE NATURE OF SPELLS
Spells do not want to be in your brain and your brain doesn’t want them there either. Spells in spellbooks, unlike scrolls, aren’t actually spells yet; they are instructions tailored for a specific wizard’s brain which results in a spell being cast. Once a spell is being cast, it has a certain momentum to it, it has lots of potential energy. Casting a spell is a little like pushing a heavy ball up the side of a ridged cone, though this is admittedly an imperfect analogy. Some schools teach the one-way arc. No matter what the theory is though, the universe wants it to settle back to a lower energy state, but precarious tiers do exist where it can rest. Many adventuring wizards (or those in other high danger/risk professions) expend much mental effort in halting this push and holding the spell in an uncast state in their minds. This effort is represented by spell slots. Such a wizard generally wants to get it close to the peak, so they can finish it with components, and the standard casting of spells assumes this, but other options are available.
Spells can still be cast (but not prepared, see above) in the standard fashion, but these rules describe two other ways of casting or preparing a spell.
Not using spell slots. This is actually the standard method of casting for most wizards, who do not practice professions where they need spells prepared to be cast at a moment’s notice. Casting a spell this way requires full tunnel-attention to the spellbook (automatically fails perception (except touch) outside of the spellbook), no other actions, expenditure of the standard spell components, and thirty minutes of time (doubled for each level above first). If the spellcaster’s attention is interrupted at any point in the process, the casting fails and they must start over.
Minimizing components. This involves using parts of the standard components during preparation, instead of leaving them as triggers for the casting. This is risky, as there’s a very real chance the spell will be accidentally cast. If material components are partially used (involved in the preparation, but not consumed, if applicable) during preparation instead of casting, any time a suitable material component the caster is carrying is jostled, they must make a DC 10+spell level Will/Wisdom save to not accidentally cast the spell. If the spell would consume the material(s), it does so now. If somatic components are partially used during preparation instead of casting, any time the caster uses their hands, they must make a DC 10+spell level Will/Wisdom save to not accidentally cast the spell. If verbal components are partially used during preparation instead of casting, any time the caster uses their mouth (or whatever it is they use to speak), they must make a DC 10+spell level Will/Wisdom save to not accidentally cast the spell.
There’s a reason why adventuring wizards do it the way they do it… this is risky business, and experimenting with magic isn’t something to be done lightly. Your best bet is to leave it to guild researchers to test and develop these things, and pay the seemingly ludicrous prices for access to stable spells. But when you can bend reality to your whim, it’s hard to take warnings of caution seriously. Compared to your mundane peers, you’re a god, right?