MuSE is the Multiple Streaming Engine developed by the programmers at Dyne (http://www.dyne.org).
Streaming is the term given to sending live audio or video over the internet. MuSE enables you to stream audio.
Perhaps a good way to understand what streaming is, is to imagine a radio station. A radio station comprises of three components - a studio, a transmitter, and the receivers that your audience has...
Above is a basic diagram showing how a transmitting radio station works. The radio studio is the source of the audio. In this space there are usually mixing desks, cd-players, minidisc players, turntables etc. Then from the studio an audio signal is sent to the transmitter. This can be sent from the studio to the transmitter by either a cable (sometimes called a "landline") or by a microwave link. Then the transmitter sends the audio via FM so that radio receivers (tuners) can pick it up and play it.
Radio works this way because it is trying to distribute the studio audio to as many people as possible. If you imagine the radio studio without the transmitter then the station would have have fairly reduced audience! Essentially only people that could fit into the studio would be able to listen. So the transmitter works as a distributer, allowing more people to connect via their radio receivers and hence the potential audience is enlarged.
This is a close analogy to why streaming exists and how it works. If you were just playing audio on a computer in your room then the audience isn't going to be so big...so, we utilise streaming to distribute this audio to more people.
The analogy is ...the computer replaces the radio studio, the streaming server replaces the transmitter, and your listeners connect by computers to the server rather than with radio receivers to the broadcast signal. Using this model we can see that MuSE is the software that sends the audio from your computer to the streaming server.
In this way you can use the program for creating a live online radio station or perhaps for event broadcasting, or other cultural or tactical projects. You can use MuSE under Linux or Mac OSX.