Low self-esteem in short, is having negative attributes towards oneself, not having firm and positive beliefs in our abilities, self-worth and lack of understanding in our conscious and subconscious mind.
This often advances and turns into depression.
It can be caused by growing up having negative image of yourself.
This often comes from our own parents, or people that surround us when we are most vulnerable in childhood. Some examples could develop in the way that all the things you ever did were never pleasant or satisfying to your parents when you were young.
You may have also been neglected of positive affection with positive feelings. It can also be caused by physical, sexual and emotional abuse which leads to psychological stress thus leading to low self esteem.