Proficiency Bonus :
At many points throughout the book you will hear the same 6 proficiency mentioned.
Untrained : You have received no training in this and are relying purely on instinct.
Novice : You have minor training, such as a few lessons with a town guard, or perhaps you were taught some basic spells through scrolls.
Trained : You have gone through enough training to be considered competent at your craft.
Skilled: You are known as an above average person of skill.
Expert : You are well known as someone who has great skill.
Master : You are hailed as one of the best of the discipline you have devoted your life to.
To apply a proficiency bonus reference the chart above. For example if you are a Master with a longsword, and are level 17 your bonus with a sword would be +14. If you were only trained you would have a +6.