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. Full definition
When browsing a vast number of stories every day, it's often hard to keep track of the important ones, or to flag them down once you've flipped past. (kevin.lexblog.com)
Funny story: I was walking down the promenade the other day after spinning and yoga (which means I was really sweaty), just minding my own business drinking a green juice... and I got flagged down by someone asking me to donate money for dying children. (thrive-style.com)
Last month, the social worker flagged me down as I was getting in my car to tell me Rob had died two days earlier. (relevantmagazine.com)