SynthFont is a program for editing and playing midi files using various sound source files like SoundFonts, GigaSampler files, SFZ files and more. VST instruments can also be used instead of a sound source file. A midi file is different from - for instance - a MP3 file in that the file does not contain any audio data.
The file is a collection of notes and instructions for how to play them. SynthFont combines this data with the audio data in a SoundFont to produce ('render') an audio version of the song.