the amount of goods and services that a country exports, minus the goods and services that it imports *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.
The trade balance is negative if a country runs a trade deficit, and positive if it runs a trade surplus.
The trade balance is an extremely important indicator of economic health for most countries. Typically (not not always) the value of the currency is strongly influenced by the trade balance also.