Streaming is the name given to the realtime delivery of audio and video over the internet. It is typically used for sending live audio for online radio stations, or perhaps sending live video from events. If you have ever listened to an online radio station you have done this via streaming audio.
M3W is a software that allows you to send live audio over the internet. You can use it for creating 'online radio', broadcasting live events, or just having some fun.
M3W is possibly the simplest software available for streaming MP3 with Microsoft Windows.