According to Wikipedia the following is the reason for the poster:
Keep Calm and Carry On was a motivational poster produced by the British government in 1939 in preparation for the Second World War. The poster was intended to raise the morale of the British public, threatened with widely predicted mass air attacks on major cities.
In 1939 and Germany began it's assault on Europe soon England was the target of the Luftwaffe, the great Battle of Britain fought in 1940 from July to October saw massive bombing raids launched against targets all across England, most notable were the attacks on London.
The Keep Calm And Carry On message was intended to bolster the British people during this dark time in history.
The poster has been brought back and has morphed into many forms. Even app developers have gotten into the game, creating apps that people can use to create they're own poster and even put them on T-Shirts.
Below is the original poster from World War 2.
