Vulnerability is the ability & willingness to expose one's own deepest truths - because other people knowing those truths cannot hurt you.
Often, the word vulnerability confused with the word weakness - however true vulnerability is the exact opposite of weakness. When you are vulnerable, you are sharing your deepest, innermost truths because you are so strong, that no one can hurt you by knowing them. It's an expression of deep inner confidence, complete self mastery, and deep trust towards the person you're sharing with.
As a visual metaphor, a warrior who is unable to be vulnerable wears full head-to-toe armor. By comparison, the monk who is so nimble that he cannot be hit by swords, does not need armor and he is aware that it will just weigh him down.
The ability to be vulnerable is an important goal of becoming shameless.