Skills in Baldur’s Gate 3 determine how well you (and every other creature) are able to perform… well, nearly everything. Skills appear in many different places throughout the game, from dialog to passive checks to saving throws. Most commonly, they are used to pass Ability Checks, where a character will apply their bonus with a specific skill to the check to add to their roll, which they will use to try to break the check’s Difficult Class (or DC).
If a character is proficient in a Skill, then they will add the relevant Ability modifier to their proficiency bonus in order to determine their Skill; otherwise their Skill will simply be equal to the relevant Ability modifier. A character is proficient in skills according to their class and background, with each class offering a unique list and amount of Skill proficiencies. There are ways to further increase skills, such as Expertise, using spells like Guidance, and gaining Advantage, but the basics of skills remains the same.
Characters in Baldur’s Gate 3 are Proficient in (and sometimes Experts in) several skills each, according to which skills they selected from those available to their class, and which skills they gained from their background.
Proficiency is simple: it is a raw bonus added to skills you are Proficient in. This bonus is added to the associated ability score for that skill. Proficiency bonus is also added to other proficiencies, such as weapons and armor, enabling you to use them to their full effect.
Expertise is an ability given to certain classes and races (namely Rogues and College of Lore Bards), which you can apply to certain Proficient skills so that you add double your Proficiency Bonus to them.
Your Proficiency (and therefor Expertise) Bonus changes as you level up, beginning at +2 for level 1 and going up to +4 by level 9.
Ability Checks with Skills
In addition to your base skill, decided by your Ability score modifiers, Proficiency, and Expertise, there are also several ways to either increase your skills temporarily, or to increase your odds when rolls.
The most obvious way to increase skills is to use the cantrip Guidance, which provides +1d4 to any roll done by a character under its effects for 1 minute. Shadowheart, who is likely to be your first companion in the game, has this cantrip automatically, and you will find that it comes in handy in the majority of dialog checks (and passive checks) in the game.
The other way to increase an Ability Check’s likelihood of success is to obtain and use Advantage, which will allow you to roll 2 dice and pick the higher result during a check, instead of just 1.
Advantage in Baldur’s Gate 3 is situational; sometimes your race or class will automatically give you advantage on certain rolls. Other times, you will have Advantage because of a piece of knowledge you possess or an attribute you’ve acquired during the game. In any case, advantage roughly doubles your chance to pass any given Ability check, and should be used when possible.