Although self-confidence can mean different things to different people, in reality it simply means having faith in yourself.
Confidence is, in part, a result of how we have been brought up and how we've been taught. We learn from others how to think about ourselves and how to behave - these lessons affect what we believe about ourselves and other people. Confidence is also a result of our experiences and how we've learned to react to different situations.
Self-confidence is not a static measure. Our confidence to perform roles and tasks and deal with situations can increase and decrease, and some days we may feel more confident than others.
Low-confidence can be a result of many factors including: fear of the unknown, criticism, being unhappy with personal appearance (self-esteem), feeling unprepared, poor time-management, lack of knowledge and previous failures. Often when we lack confidence in ourselves it is because of what we believe others will think of us. Perhaps others will laugh at us or complain or make fun if we make a mistake. Thinking like this can prevent us from doing things we want or need to do because we believe that the consequences are too painful or embarrassing.
Over-confidence can be a problem if it makes you believe that you can do anything - even if you don't have the necessary skills, abilities and knowledge to do it well. In such situations over-confidence can lead to failure. Being overly confident also means you are more likely to come across to other people as arrogant or egotistical. People are much more likely to take pleasure in your failure if you are perceived as arrogant.