A submix is a different mix of audio that can be sent somewhere you choose. It can mean that audio has different levels or that audio exists in one mix and not in another. For content creation, this usually means that you'll hear a mix that differs from what your audience hears.
Here are a few examples of where a submix might be beneficial for you:
- You want to listen to music at a different level than your audience
- Keeping your music lower (or even off) in your mix might help you hear gameplay audio better
- You want to listen to DMCA music without getting your stream/vod/video hit with a copyright claim or strike
- Your gameplay needs to be significantly louder for you than makes sense for your audience