To wink at someone means to quickly blink one or both eyes at him in an effort to convey a secret meaning. The wink is either a flirtatious action or an attempt to give a signal that only the parties involved understand.
A flirtatious wink that passes between two newly introduced people usually indicates that the person winking wants to get to know the other person better. A wink between people in a long-term relationship is a sign of affection or an alert about information those individuals share, such as an inside joke. A wink at a child is usually just a simple acknowledgment.