as for the video files/compression/etc... let me try to explain it this way.
think of a standard cassette tape. it has 2 sides, and each side has a stereo recording. so side A has 2 tracks - left and right. and side B has 2 more tracks - a different set of left and right. so a standard cassette has a total of 4 tracks. and yes, an 8-track tape does indeed have 8 tracks, since it had 4 "programs" of 2 tracks each (left+right). each time you press the program button on an 8-track player, it moves to the next pair of tracks.
sometimes there might only be music on one side of a tape, so 2 of those 4 tracks are empty. but side A will have (usually) completely different content than side B. and depending on the type of music, side A's left channel might have completely different content than side A's right channel.
the AVI file is similar. it's capable of containing multiple "tracks" of information in various formats. in the case of our dashcams, it has 3 "tracks". one is audio, one is front video, and another is rear video. there's also a 4th track of data for things like GPS and accelerometer (g-sensor) data, but we don't care about that in this instance, since it's not as easy to work with as the audio and video tracks. the problem with this multi-track setup is that a standard video player (like windows media player) doesn't know what to do with an AVI with more than one video track in it - most video files only have one audio track and one video track. VLC can kinda handle it, and of course the blacksys player can too, but it's still not ideal.
so here's how the script works. it looks inside the AVI and finds the first video track and first audio track. then it makes a copy of the first video track and first audio track and puts them into a NEW avi file called front+audio. then it does it again except it copies the second video track and makes another new AVI file with rear+audio (yes, the same exact audio track - since we only have one to work with). now you have two "standard" video files with only one video track inside, which normal video players (and editors) can handle normally. if you tell the script NOT to have audio, then it just copies the individual video tracks into their own file with no audio track. even simpler.
hopefully that makes sense without sounding too patronizing.