The FCO is the British government department that deals with other countries: ambassadors, treaties, help for British people abroad, and the diplomatic side of war and peace. The letters stand for Foreign and Commonwealth Office. “Foreign” here means other countries; “Commonwealth” is the group of countries with old ties to Britain; “Office” is just how Britain names a ministry. In speech people often say “the Foreign Office,” which sounds a bit older and more everyday than the letters. Though actually, in 2020 it was joined with the department that used to do overseas aid, and the official name became FCDO — Foreign, Commonwealth and Development Office. News and older people still say FCO a lot. It is the UK’s version of what the United States calls the State Department, not the same office, and not a world body. You meet it in news, exams about Britain, and formal writing; it is not a word for casual chat.