Flash is a bi-directional off-Tangle payment channel to enable instantaneous, high-throughput transactions. In essence, they provide a way for parties to transact at high frequency without waiting for each transaction to confirm on the public IOTA network.
You can test the demo here. You must have WebGL enabled to use this demo (Note: This is best experienced on a desktop. The use of VPNs can block the WebRTC connection).
— Dominik Schiener (@DomSchiener) September 24, 2017