A character's Fluency Bonus is equal to twice their proficiency bonus.
As a shorthand, your Proficiency Bonus may sometimes be written a "F".
Tradecraft Fluencies
As you rank up, you get the option to take additional tradecrafts, or gain a fluency in a tradecraft you already have. A tradecraft fluency is a more tailored form of the tradecraft it corresponds to. For instance, you could be a historian with a fluency in ancient Shaloban history, a tailor with a fluency in wizard robes, or an enforcer with a fluency in intimidating homeless people. When you make a tradecraft check where your fluency is relevant, you add your fluency bonus (F) instead of your proficiency bonus (P) to the roll. The GM may also tailor the results of your check with regards to your fluency.