The backstory here is demographics. Japan has a shrinking, aging population and almost no immigration. With fewer people there are naturally less things bought, sold, and produced. Japanese authorities have been trying to pump the economy by deficit spending (central government borrows money and spends it), quantitative easing (central bank buys long term private debt, increasing the money supply for short term spending) and low interest rates. The central bank has to push people to invest in the economy, because without growth driving investment returns, generally people would rather the safety of a savings account giving a guaranteed if small interest rate. Negative rates are a way to push money into the investment economy even with no growth.