The Maginot Line was built by France after World War I as a defensive mechanism against any possible future invasion by Germany. The Line consisted of "concrete fortifications, tank obstacles, artillerycasemates, machine gun posts, and other defenses" placed along France's eastern border. The theory was that the Line would give French forces time to get into position to prevent German troops from entering the territory again. Unfortunately for France, the reality did not work according to plan. The German troops largely went around the Maginot Line, overrunning Belgium, Luxembourg, and Netherlands, and invaded France in May, 1940.
In A Separate Peace, Knowles is referring to the defensive tactics that the boys at Devon raised against the looming threat of the war or whatever other threats they perceived.