To be free of worries, it is often used as a bullshit term on commercials that try to say that their product will give you "peace of mind" and somehow completly solve all of your problems
AN ADT surveillance system will give you the peace of mind that you deserve
making yourself feel good whether your standing,laying down or sitting down, concentrating on good emotions and repeating positive thoughts these thoughts are what make YOU specifically happy/feel good. Thus your peace of mind
you are your own example everybody is diverse in what makes them happy for peace of mind.
The rockin' Boston song that inspired Hannibal the Cannibal to eat brains.
Hannibal the Cannibal heard "Peace Of Mind" and thought they were singing "Piece Of Mind". Maybe they were! think about it...