When you open a video file in a media player, this code tells the application what codec to choose.
Changing the FourCC code of an AVI file only makes sense if the new codec uses the same compression method or the same compression algorithms, or otherwise you will obtain a useless video file.
This code changer has not been developed anymore since 2000
and its last version, 1.00, was designed to support Windows 95, Windows 98 and Windows ME. It allows you to edit only one file at a time.
The FourCC code is an identification key which informs your system what decoder to use in order to play an AVI file.