Dunno what an AVISynth script is though :)
It's a file that can be treated as an AVI file by most things (VirtualDub, Media Player etc.) but is actually a text file script. For example if you have AVISynth installed, and this text in a file called "aviscript.avs":
AviSource("video1.avi") ++ AviSource("video2.avi")...then it will run as a video that is video1 and video2 spliced together in a single video (as long as they are compatible). But you can do much more complicated things than splicing, you can overlay graphics, mess around with audio tracks, do professional-quality frame-rate changes, etc. It's really worth supporting - an application which dynamically generates and renders AVISynth scripts could be very powerful.