Believing in yourself is the key foundation of success in any measure. How you see yourself affects how you live and spend your day. It doesn’t mean bragging and being boastful (that is arrogance). It simply means loving yourself first because you cannot give what you do not have, and simply believing in your ability to succeed.
Self-Confidence is crucial and essential in developing ones willpower and ability to achieve. It is a positive belief that you can achieve something big in the future.
My favorite you tube vlogger alpha m. described it as unwavering belief in your ability, values and self-worth.
Never interchange it with arrogance. Arrogance is telling everybody and bragging about your achievements.