the amount of goods and services that a country imports, minus the goods and services that it exports *in a calendar year*. In 1999 Japan exported much more than it imported, so it had a trade surplus. The same year, the United States imported more than it exported, and therefore had a large trade deficit.
While Japan had a trade surplus and the USA had a trade deficit, both had something called a trade balance, which was negative for the USA and positive for Japan.
During economic downturns, political leaders become very concerned if their country is running a trade deficit, because it means that jobs are being lost to business overseas.