From the introduction:
His race, class, and ability scores define most of what a player character can do. These three characteristics don’t cover everything, however. Characters can have a wide range of talents, from the potent (and intricate) arts of magic to the simple and mundane knowledge of how to build a good fire. The character’s magical ability (or lack thereof is defined by his class. Lesser abilities, such as fire building, are defined by proficiencies.
A proficiency is a learned skill that isn’t essential to the character’s class. A ranger, for example, may find it useful to know something about navigation, especially if he lives near an ocean or seacoast. On the other hand, he isn’t likely to suffer if he doesn’t know how to navigate; he is a ranger, not a sailor. Proficiencies are divided into two groups: weapon proficiencies (those related to weapons and combat) and non-weapon proficiencies (those related to everything else). All proficiency rules are additions to the game. Weapon proficiencies are tournament level rules, optional in regular play, and non-weapon proficiencies are completely optional. Proficiencies are not necessary for a balanced game. They add an additional dimension to the characters, however, and anything that enriches characterization is a bonus. If weapon proficiencies are used in your game, expect them to apply to all characters, including NPCs. Non-weapon proficiencies may be used by players who enjoy them and ignored by those who don’t without giving unfair advantages to anyone (provided your DM allows this; he’s the one who must deal with any problems). Once a proficiency slot is filled, it can never be changed or reassigned.