One of the neatest things about mixing in Pro Tools is that you can link faders (and more) together. Then, moving one fader will move all the faders in that linked group. This is particularly useful in cases where you have a good relative blend between a number of tracks (for example, a nice balance between all the individual drum tracks), and you want to change the volume of those tracks without changing the blend. By making a mix group, you can do precisely that!
To start off, let’s get a feel for what a mix group can do. As luck would have it, Pro Tools automatically creates a group (named All), which always includes all the tracks in your session. Take a look at what it does: