Floor Broker - FB

What Does Floor Broker - FB Mean?
An employee of a member firm who executes trades on the exchange floor on behalf of the firm's clients. 

Also known as a "pit broker".

Investopedia explains Floor Broker - FB
Basically, floor brokers receive orders from their firms, which have been placed by the firms' clients, and executes these orders at the best possible prices. Floor brokers should not be confused with floor traders who execute orders for their own accounts.