The phrase "to flag down" means to signal or indicate with a flag, usually held in one's hand, that someone should stop what they are doing and come over. This term is commonly used when referring to traffic control, where a police officer or other authority figure will raise a flag to signal for vehicles to stop at an intersection or checkpoint. The phrase can also be used figuratively to mean calling attention to oneself or getting the attention of someone else by waving one's arms or using some other form of visual communication.