To single out is to focus on just one person in a larger group, either for special recognition or treatment. Your art teacher might single out one student each year to show their work at a gallery.
Even when all of your friends have been misbehaving, your parents might single you out for punishment or criticism. And during World War II, the US government unfortunately singled out Japanese-Americans, requiring them to live in internment camps until the end of the war. A restaurant owner, on the other hand, might single out an important food writer they recognize, giving them extra special service.