I'm about the lamest layman* you can find; I know almost nothing about history. So when the trailer for the new Dunkirk movie said, "The event that shaped our world," I was surprised.
It looks like a harrowing story, but how does evacuating a bunch of troops "shape our world"? What makes this particular event so important?
*That's what "layman" means, right?
So you are actually not the first to ask about this today!
https://www.reddit.com/r/AskHistorians/comments/5ibl7s/what_happened_at_dunkirk/
Here /u/thefourthmaninaboat gives a superb overview of the events of the evacuation. To summarize in the last weeks of May 1940 the British Expeditionary Force(BEF) found itself trapped in Belgium and Northern France along with many French forces due to the successful German invasion of France. To have lost these +300k men would have absolutely gutted the British Army of many of its best and most experienced troops making the tasking of continuing the war on limited manpower all the more difficult.
Thus under bombardment and attack they were evacuated in a herculean effort over about a week from the last port on the English Channel still held, Dunkirk. Vessels of all shapes and sizes were used to assist the Royal Navy, including a motley collection of civilian vessels.
That it became a sort of national effort helped build its mythos in British memory, and the ability to save the Army, and tens of thousands of French troops as well, regardless of lost equipment, did much to put a silver lining of the debacle that was the land campaign for the Allies.