This refers to weight loss being 80% about what you eat and 20% about the amount you exercise. It's not a hard and fast rule, just a general guide. It's easier to build a calorie deficit by cutting calories than through exercise but sticking to a low-calorie diet is unhealthy, unsustainable and often puts your body into starvation mode. It's much better to exercise and improve your eating habits rather than try to follow a very restrictive diet.