"I'm backing Britain" means supporting the country and its people. It's like cheering for your favorite team during a game, but instead of a game it's cheering for your country! People use this phrase to show pride and patriotism for the United Kingdom. It's like saying: "I love my country and I want to see it grow and succeed." So, if you hear someone say "I'm backing Britain," that means they are supporting their country and all the people that live in it.