An ability modifier is the number you apply to the die roll when your character tries to do something related to that ability. You also use the modifier with some numbers that aren't die rolls. A positive modifier is called a bonus, and a negative modifier is called a penalty.
When an ability modifier changes, all attributes to which that modifier applies change accordingly.
The Abilities
Each ability partially describes your character and affects some of his or her actions.
Strength (Str)
Strength measures your character's muscle and physical power. This ability is especially important in combat and athletic manoeuvres. Strength also limits the amount of equipment your character can carry.
You apply your character's Strength modifier to:
Damage rolls when using a melee weapon (including natural weapons and unarmed attacks), a thrown weapon, a sling or a bow. (Exceptions: Off-hand attacks receive only one-half the character's Strength bonus, while two-handed attacks receive one and a half times the Strength bonus. Bows have a maximum Strength bonus that they can apply to damage; for most bows it is zero so a Strength penalty applies but not a bonus.) Strength modifiers do not apply to crossbow damage.
Climb, Jump, and Swim checks. These are the skills that have Strength as their key ability.
Strength checks (for breaking down doors and the like).
Dexterity (Dex)
Dexterity measures hand-eye coordination, agility, reflexes, and balance. This ability is very important in combat, especially so for characters who take a skirmishing role, wearing light armour and/or using missile weapons. It is also important for many skills, especially of stealth and legerdemain.
You apply your character's Dexterity modifier to:
Attack rolls, both melee and ranged.
Evasion, for avoiding attacks and similar dangers (provided that the character can react to the threat).
Balance, Escape Artist, Hide, Move Silently, Open Lock, Ride, Sleight of Hand, Tumble, and Use Rope checks. These are the skills that have Dexterity as their key ability.
Constitution (Con)
Constitution represents your character's health and stamina. A Constitution bonus increases your body points and your chance of surviving critical injury.
You apply your character's Constitution modifier to:
In character creation, the formula that determines body points.
Fortitude saving throws, for resisting poison and similar threats.
Concentration checks. Concentration is a skill that has Constitution as its key ability.
If a character's Constitution modifier changes, the character's body points also increase or decrease accordingly.
Intelligence (Int)
Intelligence determines how well your character learns and reasons. This ability is important for a number of skills of knowledge, analysis and cunning.
You apply your character's Intelligence modifier to:
Appraise, Craft, Decipher Script, Disable Device, Forgery, Knowledge, and Search checks. These are the skills that have Intelligence as their key ability.
Wisdom (Wis)
Wisdom describes a character's willpower, common sense, perception, and intuition. While Intelligence represents one's ability to analyze information, Wisdom represents being in tune with and aware of one’s surroundings. Wisdom is important for mental resilience, perceptiveness and skills of care and judgement.
You apply your character's Wisdom modifier to:
Will saving throws (for negating the effect of mental attacks and magical enchantments, though these are largely not yet in play).
Heal, Listen, Profession, Sense Motive, Spot, and Survival checks. These are the skills that have Wisdom as their key ability.
Charisma (Cha)
Charisma measures a character's force of personality, persuasiveness, personal magnetism, ability to lead, and physical attractiveness. This ability represents actual strength of personality, not merely how one is perceived by others in a social setting. Charisma is important for leaders, negotiators, fixers and deceivers.
You apply your character's Charisma modifier to:
Bluff, Diplomacy, Disguise, Gather Information, Handle Animal, Intimidate, and Perform, checks. These are the skills that have Charisma as their key ability.
Checks that represent attempts to influence others.