The actual indictment is still sealed but, apparently, he's being indicted under criminal laws for paying Stormy Daniels hush money about an affair that he claims never happened.
What criminal laws does that violate? It seems to me that this boils down to a contractual agreement between Trump and Daniels (which I believe she has since violated). How is it any different from corporate non-disclosure agreements, except that this one was agreed to after the date of the alleged incident rather than before?
Disclaimer: IMHO Trump is lying. I think he probably did have a fling with Ms. Daniels. If true, that was a scurrilous thing to do, but on a personal level Trump is a scurrilous human being, so no surprise there. I just don't see how any of it gives rise to a massive grand jury investigation and criminal indictment.