A song is a particular arrangement of your tracks, created by muting and unmuting them over time. In other words, a song is a sequence of dubs, each of which mutes or unmutes one or more tracks at a particular time. The song can be visualized and edited in song view. It's possible to create a song entirely by editing in song view, but it's often more intuitive to record a rough arrangement in live view and then use song view to refine the transitions.
Creating a song is optional. The main advantage of creating a song is that you can "paint" with your polymeter loops on a timeline, in a completely non-destructive way. Editing the song never modifies the underlying tracks, and the tracks always maintain correct phase, no matter how much they're dubbed.
During playback, if the song view is active, the track mutes are automated, meaning tracks are automatically muted or unmuted as required by the song arrangement. You can start playback at any position, or skip around within the song while it's playing, and the track mutes "chase" to the appropriate state. Note however that a track must have at least one dub in order to be automated.
If the song exists, meaning at least one track has dubs, the MIDI file export applies the dubs to the tracks, so that the resulting file follows the arrangement.
A song may also include recorded MIDI input such as note or control change messages.