It's software that lets you stream PCVR to your headset. You're right that it's not a headset or hardware, but getting to play Half Life: Alyx completely wireless made it absolutely worth the $20 I paid for it. YMMV, but if you've got a Quest, a decent router and beefy PC hardware, I consider it a no-brainer. Plus, the PC versions of a lot of these games are just better. VRChat doesn't have as many arbitrary restrictions, Beat Saber lets you load in arbitrary maps/songs to play with, and your game purchases aren't locked in to Facebook if you decide to switch to a better headset (ie. Valve Index) later down the line.