in english:
AviSynth is a very very powerful tool for video post-production. It provides almost unlimited ways of editing and processing videos. AviSynth works as a *frameserver, providing instant and very fast editing without the need for temporary files.
*frameserver: A frameserver is an application that feeds video directly to another application. AviSynth and most other frameservers accomplish this by creating a fake file that other programs can read as if it were a very large (usually uncompressed) AVI file.
but
i learn in "something" not in mars and maybe i can explain.
avisynth is the envelop to transport the letter from the source to the encoder. the letter have all the words needed to the encoder understand how the source have to be used. is like a "how to"...do it using this and do that using another one but use this only if...etc!
in "something" is better