Stats in Pillars of Eternity 2: Deadfire are covered on this page.
Accuracy is part of almost every attack. It influences how likely the attack is to affect the target. Accuracy is defined primarily by a character's class and level but it is also influenced by Perception, Talents, and other active effects, such as spells or items.
When an attack is made, the Accuracy is compared to a corresponding defense (Deflection, Fortitude, Reflex, or Will) on a target to determine how the attack roll will be modified. If Accuracy is above the target's defense, it will be more likely to result in a Hit or Crit, less likely to result in a Graze or Miss.
When a damaging attack hits a target, the attack's Penetration is compared to the target's Armor Rating to determine how much damage gets through.
Penetration comes from the weapon or spell that is being used, but can be raised through the use of magic, Abilities, Talents, or weapon enchantments. Penetration is compared to the Armor Rating of the attack's damage type. E.g. A Fireball will always check Penetration against Burn Armor Rating. Raw Damage bypasses all armor and does not need to penetrate.
Full Penetration (+30% Damage) - Penetration is twice the target's Armor Rating or more.
Penetration (Listed Damage) - Penetration meets or exceeds the target's Armor Rating.
Under Penetration (-25% Damage per point under the target's Armor, Max -75%) - Penetration is less than the target's Armor Rating.
Stride represents how quickly a character can run in combat, with higher scores covering more ground.
Deflection is the defense used to resist direct melee and ranged attacks against the character that are not area of effect (AoE). It is defined by the character's class, level, and Resolve but may also be influenced by shields, certain weapons, Talents, and other effects from spells or items.
Recovery is the amount of time that a character spends between actions. Most actions will cause a character to enter a brief Recovery afterwards, though some may have shorter or even no Recovery time. Armor has a large effect on Recovery. Generally, heavier armor inflicts longer Recovery penalties.
Fortitude resists attacks on the internal physical systems of the character (e.g. poison, disease, etc.).
It is defined by the character's class, level, Might, and Constitution, but can also be influenced by other effects.
The Reflex defense allows characters to dodge out of the way of physically harmful AoE attacks (e.g. explosions, bolts of lightning). It is defined by a character's class, level, Dexterity, and Perception, but may also be influenced by other effects.
The Will defense opposes attacks that are mentally-based (e.g. a spell that mentally paralyzes the target). It is defined by a character's class, level, Intellect, and Resolve, but may also be influenced by other effects.
The character's class determines the starting value of defenses. For a multiclass, each defense is an average of corresponding base defenses for each class.