Muxing refers to combining video and audio streams. Mpeg2 used in dvd-video is a combination of both. Demuxing is, of course, separating the audio and video streams.
Muxman is an authoring program, and is the part of FAVC that puts the mpeg2 video into dvd-video format and allows you to set up a menu.
Batchmux is a command line program that launches Muxman.