"Sonderweg" comes from a German phrase meaning "separate path." Basically, the argument is that Germany's political development did not resemble the trajectory of political and social development in any other European country. This is a long-running trope in German historiography, something akin to the idea of "American exceptionalism" in the United States, but naturally the events of the Second World War form a sort of turning point in how we view it. Before World War II, the idea of a Sonderweg emphasized the fact that Germany had not developed the sort of autocratic government that emerged in Tsarist Russia, and that German society was especially amenable to capitalist development and modernization. Contrary to liberal notions in Western Europe, this was because, not in spite of, the fact that Germany lacked a truly parliamentary system and a belief that government existed to protect private property and other individual rights. German intellectual advances, and the rise of the German university, were especially central to this notion.
Needless to say, after World War II, historians looked for reasons why the so-called "separate path" had led the nation into Nazism. Some historians saw Nazism as the negation of much of what Germany had been before. In other words, Germany abandoned the so-called Sonderweg after World War I. Others, though, looked to some of the same developments that historians identified as positive before the war and argued that the "separate path" Germany travelled led straight to the rise of the Nazi dictatorship. They heavily emphasized the militarism of Prussia and later unified Germany, the anti-Semitism that was always latent in German nationalism, and the unity of interests between industrial capitalists and militarists. Because Germany lacked a liberal foundation of human rights, it was relatively easy for Nazis, who explicitly rejected the foundations of liberal democracy, to gain power. The tradition they argued against was not well-established.
Lately, historians have tended to reject the idea of a Sonderweg as overly deterministic and teleological, arguing that Germany's march toward Nazism was, like most developments in human history, characterized by contingency more than inevitability. Still, as one historian has recently written, it remains a "meaningful, though not necessarily accurate, contribution to historical understanding" in Germany.