More than happy to!
From the rules (page 36)
Trading. Gain +1 Trinket per Ailment Severity level, +1 Trinket for every 2 [Fair], and -1 Trinket for every 2 [Foul]. Foul and Fair cancel each other. You always receive at least 0 Trinkets.
Essentially, Fair and Foul represent ingredients that make a remedy better or worse tasting/smelling/feeling, which affects how much your patients will pay for them.
Beehive (Honey) on page 132 is an example of [Fair]. You can add this reagent to any remedy to give it [Fair 4], and increase the trinkets a remedy offers by 2:
Unlike other tags, [Fair] and [Foul] cumulatively stack. So two doses of Beehive (Honey) would give a remedy [Fair 8].
Hope this helps!
Brian