He took top secret documents when he left the white house. They don't belong to him, they're meant to be secured in the library of congress or something. Anyway, librarians tried to get them back, he said no, did a bunch of illegal shit (including flashing them around people he wanted to impress) so FBI got involved, raided his place, found more secret documents, special prosecutor was appointed who took ages really nailing the iron clad case together. Now he been arraigned in court facing federal charges. Because Trumps a dumb shit.

There's a LegalEagle video which sums it up: Trump's Bombshell Federal Document Indictment

Essentially,

• Trump took classified information during his time as president and failed to return them
• When asked to return them he failed to comply until forced
• When forced to comply, he still kept many documents the government knew were missing and lied to the government and his lawyers indicating that they were returned
• Was caught on tape admitting that he knew they were classified and that it was wrong for him to have them

The first bullet point apparently happens often (Biden, Pence, Hillary have all done similarly in the past). Trump is singular in that he didn't readily comply with the request for the return of documents and the consequent lies to the government (also illegal).