It sounds like some kind of quote "You fight with your mind, not your fists." But it has some truth behind it. Mental toughness programs are becoming more important - and Tal Sor Jaiphet, writing for the Australasian Martial Arts magazine Blitz, calls mental toughness the difference between a champion and a mediocre fighter.
Furthermore, that kind of mental toughness can go far beyond being a better fighter and into living a better life. Whether you intend to compete or whether you are fighting to improve fitness, you need to spend at least some time on your mental game and prioritize it at least as much as, say, strength training.
Another aspect of mental toughness is to keep control of yourself. You cannot afford to lose your temper during a fight - that is when people get hurt. You also cannot afford to lose it outside the ring. Trash talking is a weapon for a reason, it's intended to knock you off your game and distract you from your plan. Learning to ignore it can protect you outside the ring from internet BS and workplace bullying. Learning to deal with bad calls from referees and judges can help you learn to deal with the inevitable "bad calls" of life as well as making you a better competitor.
You can tune your body all you want, but if your mind is not in the game, then you are almost guaranteed to lose. You may lose your temper, you may fail to focus enough on winning, you may even get distracted by something in the audience. In fighting and in life, work on training your mind so you are ready to compete.