What is a Sub Group?
A Subgroup is a mix bus destination where you can route channel outputs. The audio channels can be processed at that point with effects like EQ and Compression. The Subgroup fader will adjust the volume of channels that are routed to that group. The Subgroup master fader only affects the level going into the main mix and does not affect the level going into any other post fader mixes. Each mixer is going to be a little different in how many subgroups they may have.
Here’s an easy way to think about it; your Main LR mix is technically a stereo group. The signal comes into your channel(s). You set the gain, then EQ the signal, and it is finally routed to that Main output fader (or specific subgroup faders that those input channels are routed to).