Demuxing is an abbreviation of demultiplexing. Demuxing is the process of reading a multi-part stream and saving each part – audio, video, and subtitles (if any) – as a separate stream. It is the logical reverse of the muxing process.
For example, AVI is a container that means audio-video interleave. The processing of demuxing an AVI requires separating the audio and visual components so (after being decoded by other modules) they can finally reach the speaker and monitor, respectively.