It's a feature that musicians can use to boost the audio quality when you're performing in a Clubhouse room: stereo sound and support for external USB microphones and mixing boards at their end of the broadcast.
The new mode is launching first on Clubhouse's iOS app, with Android to follow. It's good news for anyone wanting to perform but it may also elicit more interest from collecting societies and publishers in the platform from a licensing/royalties perspective. Via Clubhouse