Ability Score and Modifier

Source: D&D 5.2 SRD Rules Glossary

A creature has six ability scores — Strength, Dexterity, Constitution, Intelligence, Wisdom, and Charisma — each of which has a corresponding modifier. Add the modifier when you make a D20 Test with the corresponding ability or when a rule asks you to do so. See also "Playing the Game" ("The Six Abilities").