File IDentifier.

Basically, music and playlist information is stored into a structure on your machine. Typically, this is a pair of files: the actual binary data (playlist or music) and the associated emplode generated information (plaintext format with all of the database information that emplode reads). These are paired up as XXXX0 and XXXX1, where the "X"s represent a hexidecimal number. The file ending in zero is the binary data while the file ending in one is the text format information. The XXXX is typically referred to as a FID. You can uniquely identify any audio track or playlist from this unique identifier.

Does that help?

I am actually surprised a better definition isn't in the FAQ, but I did not see one either. At least not by searching for "FID"...