Not asking about the mechanics, but about the "lore" for the purpose of eg novel or short story.
Before, memorisation was represented in the following, IMHO coherent way:
You prepare a spell and mark it in your own mind, and can "feel" it as an entity of its own. To cast it, you find it in your mind, visualise it and release it using trigger components/words/gestures. Each spell is its own thing: if you want to cast it more than once, you have to memorise several instances.
Now, I'm not sure of how it should work. If it was presented in any Forgotten Realms novel, I've missed it.
The mechanics say that you prepare ("memorise", kind of) several spells beforehand. But then you can decide to only cast one of those prepared spells (of the same level), a limited amount of times per spell level.
How can this be justified, for a novel for example?
Best Answer
In Forgotten Realms lore, manipulation of magic is done through the Weave. The 5e interpretation of this is shown on pg. 206 in the side bar "The Weave of Magic".
Under "Known and Prepared Spells" on pg. 201:
Spell Slots represent the physical and mental exhaustion created by casting these spells (PHB 201, "Spell Slots"), which is why you can only cast so many spells between rests with the exception of cantrips, described as easier spells that are second nature to a caster.
Wizards differ from other spellcasting classes in that they completely study the Weave in a very scientific way. As you stated in your question, Wizards prepare their spells by fixing a certain number of spells from their spellbook in their mind for use during the day. The number of which is determined by a mix of the Wizard's intelligence and their study in the class (PHB 114). Wizards have a very wide array of spells at their disposal because they are masters of documenting the arcane. Other classes learn spells through a higher being or by practicing the physical aspect. Wizards are the only class in this edition who keep vast physical research notes to allow them to recreate not only spells they have experimented with but spells found on scrolls or other tomes.