Firstly I describe Flash Media Server as a powerful web media streaming operator that is compatible with Flash Player which becomes a web standard. With the same manner wikipedia's description may help me to understand what it is ;
"Flash Media Server (previously named Flash Communication Server) is an enterprise-grade data and media server from Adobe Systems Inc. (originally a Macromedia product). This server works together with the Flash Player runtime to create media driven, multiuser RIA’s (Rich Internet Applications). The server uses actionscript 1, an ECMAScript based scripting language, for server-side logic." from wikipedia description.
Flash Media Server 3 will be released officially (around February 8) with the approved features like high-performance live streaming, double streaming performance, approved industry standards, new plug-in architecture and etc.. Although free developer edition can be downloaded first below link.
see links below: