The Maginot Line was a series of fortifications and defenses that France constructed along the German border after World War I. The fortifications did not extend along the border with Belgium, a French ally. As a result, when Germany invaded France in 1940, it also invaded through the Low Countries of Belgium and the Netherlands, as well as through the Ardennes Forest which lay north of the French defenses. Thus the Germans were able to avoid assaulting the Maginot Line directly by simply going around it.
Since then, the term Maginot Line has been used as a metaphor for something that is confidently relied upon despite being ineffectual.