He's pleading guilty to making false statements to the FBI. It's a plea deal, in exchange for dropping the many other things he could have charged with. This means Flynn is cooperating with Mueller to get someone higher up on the foodchain. That could only mean two or three people, Sessions, Pence, and Trump, with a sidedish of Jared.
Wonder if Hillary will bring a crowd over to his sentencing hearing to cheer "Lock him up! Lock him up! Lock him up!"