It's a state of being emotionally exposed in way that includes uncertainty. Like sharing emotions when you're unsure how the other person will respond. In this case, it might mean opening up more to an acquaintance as a way to develop a friendship but being unsure how they would receive that or reciprocate.
It's simmilar to physical vulnerability but with emotions. Displaying emotions in such a way that someone could - if they were so inclined - emotionally exploit and attack you. For example, opening up about your greatest fears can open you up to ridicule or someone pranking you.
