Spell casting 101
Spell Slots
All spell casters (with the exception of Warlocks, which we'll ignore for all of this) have a certain number of "Spell Slots". The number of slots the caster gets and the level of those slots increase with class leveling.
A 1st-level Sorcerer has 2 Spell Slots, both at 1st-level.
Spells Known/Prepared
There are two terms that mean roughly the same thing; spells that are "known" and spells that are "prepared". I'll explain the difference shortly.
Different casters learn spells in different ways:
For instance, Clerics, Druids, and Artificers have access to all the spells for any level they have appropriate Spell Slots. So at 1st level, they have access to any 1st-level spell from their class spell list, but they can only prepare a certain number of spells based on their level and their spell-casting ability score. They can change out the spells they prepare each night after a long rest.
Wizards are similar in that the number of spells they can prepare is based on their level and Intelligence score. However they don't automatically know all of the spells for a given level. Instead they can chose 6 spells at 1st Wizard level, and get 2 more each Wizard level thereafter. Similar to the above, Wizards can change out their spells every night, but only from the list of spells they have learned. So that would be the original 6, plus the 2 chosen per each additional level, plus whatever spells they manage to find/buy and inscribe in their spell book.
Bards and Sorcerers have access to all the spells for the appropriate level. However, they can only know a fixed number of spells per level. Additionally, they can only swap out the spells they know when they level up in that class.
And that is the key difference between prepared and known; when a class prepares spells, they get to pick and choose on a daily basis, whereas when a class knows a spell, they can only modify the list of spells while leveling up in that class.
A 1st-level Sorcerer knows 2 spells (and 4 cantrips). They also only have 2 1st-level Spell Slots in which to cast those spell in any way they want per long rest; either the same spell twice, or one spell each. At 5th Sorcerer level, they have a total of 6 Spells Known with a mix of 1st and 2nd-level spells. They also have 4 1st, 3 2nd, 2 3rd Spell Slots. This makes 9 Spell Slots, but slots have a power level. You can only cast 3rd-level spells using a 3rd-level Spell Slot. You could also cast a 1st or 2nd-level spell using that same 3rd-level slot, and depending on the spell it may become more powerful.
There is no direct correlation between the number of Spells Known/Prepared and the number of Spell Slots. You could have all 1st-level spells; although you should really have a good spread of all available levels.
This was alluded to earlier, but bears repeating. When you learn spells (known or prepared), they are not tied directly to your Spell Slots other than certain spells will require higher level slots. In prior editions of D&D, the slots were tied directly to the spell. So if you wanted to cast a spell twice, you would memorize it twice. This is no longer the case. For instance, when my Sorcerer was 1st level, he knew magic missile and sleep. On any given day he had two 1st-level Spell Slots. He could use those slots to cast sleep twice, magic missile twice, or each once. And he did not have to make that distinction prior to actually using the slots--Only when he decided to cast a spell.
Metamagic and Sorcery Points
Sorcerers also have something called Metamagic. It's a feature that allows them to shape spells in ways no other class can: improve damage, cast without moving, cast a spell as a Bonus Action, etc. But all these features require Sorcery Points. Sorcerers, starting at level 2, have a number of Sorcery Points available to them on top of having Spell Slots. Each Metamagic option takes a certain amount of Sorcery Points. These too are not tied to specific Metamagic options; you can pick and choose how to use the points as you need them.
Flexible Casting to tie it together
Flexible Casting lets you convert Spell Slots into Sorcery Points or from Points into Spell Slots.
Let's say you've used up all of your Sorcery Points, but you want to do a Subtle Spell soon. As a Bonus Action you can use Flexible Casting to convert an unused spell slot to create some Sorcery Points. So if you have a spare 2nd-level Spell Slot, you can convert that into 2 Sorcery Points allowing you to cast a couple Subtle Spells.
Alternately, if you have extra Sorcery Points, and need to cast an emergency 2nd-level spell. As a Bonus Action you can change 3 Sorcery Points into a 2nd-level Spell Slot and cast a 2nd-level spell.
Recap
- Just because the 5th-level Sorcerer has 4 1st-level Spell Slots does not control whether on not the character should have 4 1st-level Spells Known.
- Flexible Casting is just a method for converting Sorcery Points to Spell Slots and back again. It gives the character "flexibility" in how they use their magic for the day.
Think of all of these things as buckets; Spells Know, Spell Slots, Metamagic options, and Sorcery Points. So each level you get to have a bucket of Spells Known. You have a bucket of Spell Slots to cast those spell. You also have a bucket of Metamagic options and a bucket of Sorcery Points that can use as needed. Flexible Casting can convert things from the Spell Slots bucket into things for the Sorcery Points bucket or Sorcery Points bucket into Spell Slots bucket.
And when it comes time to cast a spell, you grab a spell from the Spells Known bucket and match it with a Spell Slot. And maybe you grab a Metamagic option and use some of the Sorcery Points to apply it. Put everything together, then cast your spell.
