Innate Spells
When you gain an innate spell, you become trained in the spell attack modifier and spell DC statistics. At 12th level, these proficiencies increase to expert. Unless noted otherwise, Charisma is your spellcasting attribute modifier for innate spells.
If you have an innate spell, you can cast it even if it's not of a spell rank you can normally cast. This is especially common for monsters.
You can't use your spell slots to cast your innate spells, but you might have an innate spell and also be able to prepare or cast the same spell through your class. You also can't heighten innate spells, but some abilities that grant innate spells might give you the spell at a higher rank than its base rank or change the rank at which you cast the spell.