It measures the trade balance between imports and exports. Exports greater than imports shows a trade surplus while exports lower than imports shows a trade deficit. Any fluctuation in figure significantly affects the domestic economy of Japan. Japan economy heavily relies on trade surplus, which is a positive figure for economic growth. The data of Merchandise Trade Balance Monthly is released monthly usually in the fourth week of reporting month by the Ministry of Finance and Customs Office Japan.
Reading higher than expected is taken as positive/bullish for JPY while reading lower than expected is taken as negative/bearish for JPY traders.