CF: Spell Library / Prepared Spells

You have a vast array of spells you can tap into, called your Spell Library. You cannot use them all, but you can prepare a number of spells per day equal to your level in the class that grants this feature + your spellcasting modifier (Intelligence for Wizards, Wisdom for Clerics and Druids, Charisma for Paladins, etc).
Adding Spells to your Spell Library goes along with the Spell List Collection or the Spell List Understanding common features, which you will get along with the Spell Library feature, unless you prepare your spells in a different manner.
You prepare spells by doing the Preparing Your Spells activity, with said information being repeated here.
You prepare the list of spells that are available for you to cast. To do so, choose a number of spells from your source of spellcasting (either a Patron/Deity/Higher Power with Clerics and Druids, and a Spellbook for Wizards) equal to your Spell Collection class level + your caster modifier (minimum 1 spell). The spells must be of a level for which you have spell slots or spell points.
For example, if you're a 3rd-level wizard, you have four 1st-rank and two 2nd-rank spell slots. With an Intelligence of 3, your list of prepared spells can include six spells of 1st or 2nd rank, in any combination, chosen from your spellbook. If you prepare the 1st-rank spell Magic Missile, you can cast it using a 1st-rank or a 2nd-rank spell slot. Casting the spell doesn't remove it from your list of prepared spells.
You can change your list of prepared spells when you finish a long rest. Preparing a new list of spells requires time spent studying your spellbook and memorizing the incantations and gestures you must make to cast the spell: Around 10 minutes, or a Swift Rest's time.