What happened at Dunkirk (France) in 1940 was the evacuation of the British Expeditionary Force (and a fairly large number of French and other continental European soldiers to England. This evacuation was hugely important because it allowed the British army to continue being a part of the war and it allowed for the creation of a "Free French" force.
In the spring of 1940, the Germans had swept across Western Europe in the "blitzkrieg." The British Expeditionary Force and the French Army had been unable to do much to resist. By late May, the remnants of these forces were trapped in Dunkirk. If the Germans had either killed or captured these people, the war might well have been over.
However, the British were able to evacuate 338,000 soldiers from Dunkirk. This was one of the most significant events of WWII.