Too many people find themselves succeeding at their work but ultimately unfulfilled because they're doing what they're good at as opposed to being good at what they do. "Doing what you're good at" can produce satisfaction from recognition and reward but you still feel hollow. You're a great lawyer, or doctor, or product manager, yet you don't identify with your profession as something which is true to your inner self. You say things like "if i had a lot of money i'd really pursue my goal of being a writer," or a scientist or a history professor.
Instead "being good at what you do" means embracing these desires and pursuing them with confidence, knowing that you will find a degree of success because you're passionate, smart and hardworking.
Doing what you're good at is narrowing. Being good at what you do is expansive.