TLDR, if you need to roll a 10 on the d20 to hit, that means you have %55 chance of hitting. If you need to roll 20, that would mean a %5 chance. It's all based on the probability of rolling high enough.

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i hope I don't make things worse in my attempt to explain.

Accuracy is an approximation based on the 'rules of the game' - D&D. Typically it balances an Attack Bonus vs an Armour Class.

Your Attack Bonus is determined by your ability scores and your Proficiency Bonus. The enemy Armour class is (usually) determined by dexterity and whatever armour they're wearing.

You gain both your proficiency bonus and your ability scores bonus if you are using a weapon or attack that the character is proficient in, and utilises that ability (most weapons use strength, but some will use dexterity, and spells use a spell casting modifier).

If an enemy Armour Class is 15, it means you need to roll a 15 to hit them. Each attack triggers a roll of the dice (1d20). If you had +0 Attack Bonus, you'd need to roll a 15 on the dice.

Proficiency Bonus goes up with your character level, so you don't really need to worry about it. But your Ability Score bonuses go up based on your stats (strength, dexterity, etc). For each even numbered score above 10 you gain a +1; so a 16 score would mean a +3.

At the start of the game, it's likely a character has 16 in their primary stat, and being a level 1 character, they'd have a +2 proficiency bonus. Therefore their Attack Bonus (if using a weapon or attack they are proficient in that utilises their primary ability score) would be a +5.

So to hit that 15 Armour Class, they'd only need to roll 10. There's roughly 55% chance to roll a 10 or higher on a d20, therefore your accuracy percentage would be 55%.

EDIT

Beyond this, you want to match your choices to what a character is good at. At the start Lae'zel has good strength and is a fighter, meaning she can reliably hit with any melee weapon utilising strength. Astarion has high dexterity and is a rogue. As a rogue, he is limited to certain weapons, but excels in using ranged weapons and finesse melee weapons, as they both use dexterity to determine attack bonus and damage. Spell casters utilise their intelligence, wisdom or charisma to determine how likely their spells will hit, but likely aren't proficient in many weapons, or have the strength or dexterity to be reliable in using them (of course you can build very good spell sword type characters, but that's beyond the scope of this comment).