JK = Joshi Kōsei 女子高生 and means “high school female student”.
JK business = Compensated dating with adolescent girls.
A typical scenario of JK encounters = A girl gives out leaflets inviting for a “JK walk” (JK osanpo) or “walking date”, typically in Akibahara. Then the uncle pays the girl to walk together, have conversations and such.
It doesn’t involve sex “per se” or by default, but mixing an uncle with a very young girl… what could ever go wrong?? = A lot. Like this JAV video fantasy tends to suggest.
That’s why lately Japanese have been cracking on this… apparently.