To be a Fern tamer is to be sexualy inclined to fuck those who have furry genitals.
Scenario#1
Alfy- yo, wuttup, CAN I see your bush FAM?
Dan- AYOO WTF!?, what are you, some kind of fern tamer!?
Scenario#2
Furry#1- You know you wanna stick it up my fat furry ass you fucking fern tamer~
Furry#2- *gets horny*