If you want to be more successful — at anything — than you are right now, you need to know yourself and your skills. And when you fall short of your goals, you need to know why. This should be no problem; after all, who knows you better than you do?
And yet your own ratings of your personality traits — for instance, how open-minded, conscientious, or impulsive you are — correlate with the impressions of other people (who know you well) at around .40. In other words, how you see yourself and how other people see you are only very modestly correlated.
Who's right? Who knows you best? Well, the research suggests that they do — other people's assessment of your personality predicts your behavior, on average, better than your assessment does.