Welcome to SimV...

The first addon at the top of that page is the 'FS9 to FSX AFCAD Converter'. Basically, Afcad files have airport information such as the runway, taxiway, and parking layout for a given airport. Most airports only have a few parking spaces for AI aircraft, so when you download AI airlines and such, there can be a shortage of parking. This program allows you to take Afcad files for FS9, and convert them to FSX, saving you a lot of time searching, or making your own Afcad files specifically for FSX.
I hope I was able to answer your question. If you have any more questions, don't hesitate to ask