Blitzkrieg (German for 'Lightning War') was a new way of fighting that the Germans developed in the 1930s. In the past armies had relied on horses and so advances were slow. The Germans saw that using motor transport would speed up any advance.
They were right. They had built motor roads (autobahnen) right to the borders with neighbouring countries, so they were able to get supplies to their troops quickly.
Advances right across Western Europe took just a few weeks as a result of this new way of attacking.