what is muxing and demuxing

They're short for multiplexing and demultiplexing. Multiplexing means combining different types of data in a single stream or file. On this board, they're talking about combining the video and audio data in a single file. Demultiplexing means splitting the video and audio out into separate files.

关于音视频的一些知识（demux、filter等）

http://blog.csdn.net/haomcu/article/details/7072707

Mux? Demux? Remux? Huh?

http://adubvideo.net/informative/mux-demux-remux-huh

mux：视频/音频封装

demux：解封装视频/音频