Self-confidence allows you to have positive yet realistic views of yourself and the situations in which you are involved. If you have self-confidence, typically you do not fear challenges, you are able to stand up for what you believe, and you have the courage to admit your limitations.
Most of us have areas in our lives where we feel quite competent while at the same recognizing areas where we do not feel at all confident. Having an accurate sense of self-confidence means you avoid behaving overconfident or reckless. It means you are not afraid to take risks on tasks that you are able to do and you do not get paralyzed by the fear and anxiety when faced with things you want or need to do.
People with high self-confidence typically have little fear of the unknown, are able to stand up for what they believe in, and have the courage to risk embarrassment. Losing confidence is no longer trusting in the ability to perform. It may be reasonable as the result of past failure to perform, or unreasonable, because one "just has a feeling" about something or is having doubt.