how is that working for you?
It means to train your body and your mind. If one is weak, the other one cannot be useful as well.
Prime example for an unbalance would be Stephen Hawking as "body is weak, mind is strong". His intellect can make up for many things his body can't do for you, but i think he'd trade some part of his IQ for a better body anytime. Even if not, few people are blessed with is intelligence.
A healthy body and a healthy mind, this is what makes you get along very good.