This was originally introduced by fanum who would regularly show up on stream with Kai cenat. It was sort of seen as a meme originally. However it seems to have been adopted by gen z now as a term that means a tax by friends on one another or by fans on their idols when they share their creations directly with the idol or steal something from the idol or an event to remember them by.
Scenario 1
Ashley "Did you get your fanum tax at the concert last night?"
Josh "Nah. I was to into the show."
Scenario 2
David Duchovny "Wow! Did you paint that yourself? You must really love the xfX-Files!?"
Donna: "YA! I loved the show and so I made this for you as my (fanum)tax"
David Duchovny: "Wow! Thank you so much you. I appreciate the gift!"
Scenario 3:
Patrick: *eating hamburger and fries while at lunch at school*
Jessi: "hey don't forget the fanum tax"
Patrick: "You first"
*they both trade parts of their lunch*