You might fill several different roles on any day.  In one day you could be a sister, daughter, female, friend, niece, granddaughter, student, athlete, musician, etc.  Your self-esteem is based on how satisfied you feel about your performance in these different areas.  You may think you're a great soccer player but you're not very good at the piano.  You may think that you're very smart but not very pretty.  You may think you're a great friend but could be a better sister.  All of these are factors in your self-esteem.

If a girl has positive self-esteem it means that she is satisfied with herself.  She might think that she could improve in a few areas, but she is not stressed out about them.  She knows what her strengths are and she exceeds in those areas.  She also knows what her weaknesses are and she tries harder in those areas, even though she knows she'll never be the best.  She doesn't let the fact that she can't be the best at everything bother her because she knows that she has her good points, just like everyone.

If a girl has negative self-esteem it means that she is not satisfied with her overall self.  She might think that she has no strengths because her weaknesses seem so great.  She might not be a good athlete and she lets that be in the front of her mind, forgetting that there are other things she is good at.  Often we focus on the negative and ignore the positive, because the negatives seem like a bigger deal.  When one thing bother her a lot, she usually looks at other things in a negative light. 

Most girls go through periods of negative and positive self-esteem, and it's a normal part of living in today's society.  Yet if you seem to have a negative self-esteem more often than you have a positive one, that can be a problem.  There are several things you can do to try and change that.  It's normal to feel stressed out that you can't do everything all the time and be the best, but the truth is-no one is the best at everything all of the time.