The specifics of the deal have not been announced, but a note posted by Harmonix's John Drake (but attributed simply to Harmonix) states...
Though there's an investment firm backing them up, this essentially sounds like Harmonix is once again in control of Harmonix, something that sounds like a pretty good deal to me.
We’re excited to be returning to our roots as an independent and privately owned studio. As for what it means for you, the DLC schedule marches on for Rock Band, we will continue our support of previously released titles and we’re hard at work on some unannounced projects that we think you’re going to be pumped about.
Here's a loosely related video that I'm not going to bother explaining because it would take too long and the end joke wouldn't actually be that funny.