The term in the title is a military and political term describing the deliberate placement of civilians in or around combat targets to deter an enemy from attacking those targets or to place hostages around and in front of perpetrators as they walk passed armed policeman for protection as they escape.
The trivia item below may give away important plot points.
The beginning scene was borrowed from a real life event. On July 2, 2005, a man named Joseph Edward Duncan III was in a Coeur d'Alene, Idaho Denny's with a 9 year old girl that he had kidnapped. Other patrons recognized the man and girl and called the police. The staff was able to stall Duncan long enough for police to arrive. Unlike the ER storyline, the real life event did not involve a shoot out between Duncan and the authorities.