Input Gain / Pre Fade Monitoring
This is a great tutorial on level setting.
This is a hidden feature in some DAWs and it is so important! When you have an audio track or a VST instrument with a very loud volume that is in the red many people think just by lowing the volume can solve the problem.
This can be the case sometimes, but the input will always be overdriven no matter what you master fader does. This can lead to a nice effect being in the red, but also could lead to dirtiness in your mix.
This track allows you to be aware of your volumes so you can get a clean and crisp mix with headroom.