SoundFont is a file format for sample based instrument sounds. You will need a SoundFont to use [FluidSynth].
If you just need to play General Midi files, these SoundFonts are known to work well with FluidSynth:
More download sites for SoundFonts:
The SoundFont format was originally created by Creative Labs and EMU Systems and used in the SoundBlaster AWE 32 and later cards. There are now many other hardware platforms and software synthesizers supporting this format. SoundFont 2.0 and later are open formats and the specification is freely available.