It's not a crime. It is distasteful but if someone wants to "spill the beans" you either A. go to court, spend a small fortune defending yourself and everybody gets to hear about it anyway or B. persuade them with a "settlement" up front.
The "fraudulent business activity" is a carefully worded spin on the facts of the matter which appear to be relating to how a dodgy accountant made those payments after Trump told him to use cash. The accountant committed fraud (I don't know the details) and was prosecuted/jailed for it. It is unclear what Trumps involvement would have been, one assumes he just bankrolled the accountant and left him to get on with it.
Skip forwards a decade and suddenly the Biden admin have decided Trump is a problem and somehow this payment needs to be reinvestigated as an election expense. It was not declared as an expense. I assume thats because the payment was made after Trump was in the Whitehouse.
It wasn't just a single reimbursement, apparently it was effectively paid off by employing the accountant on a retainer, hence the "34 counts of business fraud". Sounds bad right until you realise it's just the one thing and that thing is bollocks anyway.
The case will go nowhere of course but that doesn't matter, sharper minds than mine will ensure it carries on well into the election cycle so that the mainstream media will have something to smear Trump with. Somewhat ironic that this Trump hush money is being touted as "election interference" when the Biden admin are literally guilty of actual election interference!