"Old Man at the Bridge" by Ernest Hemingway is a story set during the Spanish Civil War, a conflict occupying the years from 1936 to 1939 in which the conservative and protofascist Nationalists were fighting against the liberal Republicans. It was an extremely violent conflict with atrocities perpetuated on both sides--but especially by the Nationalists. Hemingway worked as a reporter for the North American Newspaper Alliance during the war.
A pontoon bridge is a temporary structure using air-filled floats or small boats to support a path across a river. Because these structures are cheap and quick to erect but flimsy and unable to bear large weights, they are normally used only in emergencies or military operations.
The people crossing the bridge are fleeing from a war zone. They know that if they are trapped when the fighting starts, they may well end up as "collateral damage." They are residents of a small village and fear for their lives.