Loving yourself enough to look out for your best interest in the long-term.
This means not allowing others to take advantage of you or walk all over you. It's also about learning to accept your imperfections as they are what make you unique. You have to invest time into yourself learning how to get the most out of life while you're on this planet and then share what you've learned with others. Being happy with who you are without the need of validation from others is the highest form of self-esteem in my opinion.
I guess it's a thin line between having a high self-esteem, narcissistic, and arrogant! :)