You set a target in your life. (That usually happens when you are in high school or earlier. The earlier the better.)

If you are smart, the target should be achievable.

Most people with average intelligence usually reach their goals in life. (Making decent money, having a decent salary job, wife&kids, a yacht etc)

If you are smarter, you achieve your target faster than the others. A lot faster. That makes the real difference. I am not that smart but I can identify a smart guy instantly. Most people don't have that talent. :p