When someone briefly closes one eye while looking at you with the other eye, they may be "winking at" you. Winking is a gesture that suggests that the winking person is sharing some kind of secret with you.
People sometimes use a wink to suggest that there's some kind of sexual attraction between them and the person they're winking at.
You can also wink at someone to show that you're lying to another person. For example, you might tell your brother that the dinner he cooked was really good, while winking at your brother's children to show that it actually wasn't very good.