This advertisement appeared in British newspapers during the Second World War, shortly after the Battle of Britain - a series of intense attacks on British cities by the German air force. The advertisement was paid for by the British government.
The Union flags, either side of the royal coat of arms, are the national flags of other countries and colonies of the British Empire who were allies of Britain in the war. The text describes the British Empire as a Commonwealth, a 'family of free nations'. Some rule themselves, others are not yet independent from British rule.