This is an overused cliche that means nothing.
Defence requires a fullback that knows how and where to direct defenders returning to the line.
An understanding of when the space and numbers dictate jamming up and in or sliding.
This also needs to be practiced until the team reacts to situations the same, rather than one jamming whilst another slides.
Knowledge of the abilities of the guy next to you in order to trust he can complete a tackle or if he will require help which will leave an opponent unmarked.
Hours of gym and wrestling in order to have the physical ability to stop your opponent and the technical skills to work with a teammate to put an opponent on their back.
That is just the preseason work.
It then involves doing the hours of video each week to know the oppositions attacking structures and then hours practicing defending those structures on the training field.
This base is all needed before having the right attitude of working a little harder to cover your inside shoulder during a game.