Using Abilities

Six abilities provide a rough description of a creature's overall characteristics and capabilities. Strength, measuring physical power Dexterity, measuring agility Constitution, measuring endurance Intelligence, measuring reasoning and memory Wisdom, measuring perception and insight Charisma, measuring force of personality.
The abilities lay the groundwork for how the character interacts with its surroundings. Each ability has a score ranging from 1-30, with 10 representing the average person and 20 marking exceptional skill - whereas legendary monsters and divine beings can reach scores of up to 30. The score of an ability determines a modifier. This modifier directly influences the rolls which you make (like ability checks and attack rolls) like this:

The effect Advantage or Disadvantage can be bestowed by various skills, and means that you must make 2 rolls, and use the higher or lower result, respectively.
The so called proficiency bonus is determined by level progression (see class overview) and generally is only applied once.