The Sonderweg Thesis is an argument about German history that purports to explain why (among other things) Nazism or a similar political system was inevitable in Germany. Before World War II, there was a slightly different Sonderweg Thesis that was based on the idea that Germany had taken a special path (this the meaning of "Sonderweg") to development.
The earlier Sonderweg thesis held that Germany had taken a middle way that was not like that of any other European country. Germany had avoided total autocracy such as was present in Russia. At the same time, it had avoided going too far in the other direction and embracing democracy like the British had. German historians felt that Germany's path was superior to those of other countries.
After WWII, the thesis changed. It still emphasized how different Germany's past had been. But now it cast this in a more negative light. It now argued that things like the continuing power of the hereditary aristocracy and the glorification of the military had pushed Germany towards Nazism. From this point of view, the special path was one that led to disaster.