What does it mean to be humble?

When I first started exploring Christianity I kept hearing about the importance of humility, yet I didn't know what that meant -- I thought it meant to have low self-esteem or something along those lines. I wish I'd read this back then, because it's a great summary. An excerpt:

Humility is not the trait of thinking of self as being of less worth. Humility is not thinking of self at all, whether good or bad. Humility is thinking of others and seeking their advancement...Stop considering your humility and use your energies and confidence to help others.

Read the rest here.