People with a growth mindset believe that they can improve with effort. They outperform those with a fixed mindset, even when they have a lower IQ, because they embrace challenges, treating them as opportunities to learn something new.
With a fixed mindset, you believe you are who you are and you cannot change. This creates problems when you're challenged because anything that appears to be more than you can handle is bound to make you feel hopeless and overwhelmed.
Personality traits form at an early age and are fixed by early adulthood. Many important things about you change over the course of your lifetime, but your personality isn't one of them.