What is MIDI, anyway...
and what the heck are SoundFonts?
The digital equivalent of sheet music, MIDI was commonly utilized by the video games of the 80's-90's. MIDI doesn't have any sound information in it per se, therefore the fidelity of the samples utilized by your MIDI synthesizer determines the quality of your MIDI sound as much as the composition itself.
To put it simply, a SoundFont is a library of instrumental sound samples, instructions and parameters which, when loaded by a compatible synthesizer (either implemented in hardware or in software), will dramatically change the sound of MIDI output. It follows that your choice of SoundFont can have a noticeable impact on your classic video game experience. Especially if you're an audiophile like me.