Basically it is a music program that combines Spotify with Twitter.
Currently it is in the public testing phase, but invites only (and the UK only).
The premise of how it works, is basically you 'Flow' or 'Reflow' songs to your followers that can listen to the song in full once, and then any 30secs of that song again. The more people you follow the more full songs you can listen to.
The big thing about this is if anyone buys the song that you flowed to them, you get a percentage of the sale.
P.S. I have 5 invites if someone from the UK wants to have a look.