How to make parts play the entire score, but the selected tracks louder?

I know there is a way to make it so that if you're playing a part of a song, it will play all the other instruments in your song but with the selected instruments playing louder so you can hear them better. (e.g. you selected a part that contains only the vocal tracks as the sheet music, but you want to hear the accompaniment as well) Not sure how else to word it. How do you make this happen?