Fastest Fish sets up a peer to peer connection between the file sender and the reciever. This means in most cases the file is being transfered direct between the two devices rather than through a server.
Using peer to peer to transfer your files means you can transfer large files without running a server with high bandwidth costs.
The direct connection also allows your devices to transfer files at the miximum speed that your networks can handle.
Chrome - Full support
Firefox - Partial support
Edge - Partial support
Internet Explorer - No support
Partial support refers to a limited file transfer size due to missing browser features.