Self-esteem is know as self worth or self respect, and has to do with how much you like your self, how much you value yourself, also the way you feel about yourself (you physical appearance, what you think people think about you, which in my opinion really doesn't matter but in their head it means so much)
Low Self-esteem is when you don't value yourself, you always feel other people a better than you, that you can't amount to anything but that it's just a lie, it's all in your head, and you need to break free for your own good.
Many people today are in a bad relationship or have a very terrible job, they know that but they still stay anyways, because they have low self-esteem, they don't believe in themselves, they don't think they can do better than their currently relationship or job, they get exploited and taking advantage of.
The bad thing is that they keep coming up with excuses to justify them being in that bad situation, no matter how hard you try to help them, if they don't want to break free there is nothing you can do for them, because they will keep going back to that bad relationship or job.
To break free from the low self-esteem problem, you have to realize the reality that you're better than your current situation and it doesn't define who you're.
start thinking positive about yourself, start daily positive affirmations morning and night, until you start believing it.
Accept that you're not prefect and nobody is prefect, start loving yourself for who you're.
Like @lookagain said in Are you willing to fight for your freedom?:
Freedom is without a doubt the most precious thing a person can have, depriving yourself of freedom is like a bird trapped in a cage, where fear and sadness are part of the daily routine.