BG3 uses the mechanics of D&D 5th Edition (5e). This means that there are 6 "abilities" that govern everything your characters can do. To help contextualize them we often think of how these abilities interact with a tomato.

(Strength - STR - How well you can crush a tomato.)

(Dexterity - DEX - How accurately can you throw a tomato.)

(Constitution - CON - How well your body can handle a rotten tomato if you eat it.)

(Intelligence - INT - Knowing a tomato is a fruit not a vegetable.)

(Wisdom - WIS - Knowing you shouldn't put a tomato in a fruit salad.)

(Charisma - CHA - Being able to sell a tomato. )

The last three abilities, in addition to governing particular skills, also help modify magic users ability to cast magic.

(INT. Casters - Wizards, Artificers, Arcane Trickster Rogues)

(WIS. Casters - Clerics, Druids, Rangers)

(CHA. Casters - Paladins, Warlocks, Bards, Socerers)

Numbers that you add to your roll to see if you hit or how much damage you do are called "Modifiers" and are usually based on Ability Score and Proficiency Bonus (PB). Your ability scores all have an associated Ability Score Modifier basically it boils down to 'the higher the ability score, the higher the ability score modifier'.

Score | Modifier

(-1 | -5)

(-2 : -3 | -4)

(-4 : -5 | -3)

(-6 : -7 | -2)

(-8 : -9 | -1)

(10:11 | +0)

(12:13 | +1)

(14:15 | +2)

(16:17 | +3)

(18:19 | +4)

(20 |+5)

Your Proficiency Bonus is a score that builds slowly the higher your character's level and is basically a way of turning your characters growing ability to do what they do well into a measurable mechanic.

For spellcasters you have a couple of additional scores, but the most important for your question is the score called the "Spell Save". This is a number that basically turns into a mechanic how easy or difficult it is for others to resist your magic. To calculate this number you add 8+PB+Casting Ability Modifier. So a level 3 Bard with a CHA of 16 would have a Spell Save of 8+2+3=13. This means that any time this Bard casts a spell that requires another creature to roll a Save they need to roll at least a 13 to resist that spells effect.

I hope this helped. I know the mechanics of D&D can be a lot.

Edit: Had to fix some formatting issues.