[jckt]

I think he/she means that normal exercise, or rather the typical amount of exercise that people do, is insufficient to burn the calories in a normal diet. So while the exercise you do is healthy, you shouldn't be expecting weight loss, though at the same time you should expect less weight gain (which I wouldn't consider as weight loss).

[SiVal]

No, it's that people who exercise more tend to eat more, even when their primary reason for exercising is to lose weight. They either don't think about it or overestimate the calories burned by their exercise and allow themselves to eat a bit more, mistakenly believing that the net result is still negative calories.

Moderate exercise is good for you whether or not you want to lose weight, but if weight control is one objective, you need to be more careful than most people realize to avoid eating more when you exercise more.