The British government in 1939 during the beginning of the Second World War, intended to raise the morale of the British public in the aftermath of widely predicted mass air attacks on major cities by posting signs that read “keep calm and carry on”. 
The intent was to strengthen morale in the event of disaster.
If we are to stand strong as a nation, as a people we need to embrace this and not show panic and fear, but remain calm and level headed so we can think clearly and react (not over react) in a stressful situation.