Whereas the layout block \layout{} is used to generate a pdf, the midi block \midi{} will generate a midi file.
Depending on what software application you are writing LilyPond script in, this midi file will either be playable through that particular software (for example Frescobaldi), or you will have to open that midi file with some other program. Midi files are great because they can be opened with a large variety of music programs (for example MuseScore).