Simply put, the step means that as commercial banks park their assets at ECB, they now face a fee. The idea is to nudge banks into placing money in real economy, lending it out to enterprises as well as consumers. Also, the decision comes from the bag of measures that might provide a boost to eurozone’s lacklustre economic growth. By making banks pump out more money of their reserves, ECB is like a doctor taking care of his patient’s circulation.
ECB’s move also reduces deflation hazard. European Central Bank mission statement is to guarantee price stability and keep annual inflation at two percent. At the moment, however, eurozone has it a lot lower. Should inflation keep sinking and plunge below zero, prices – instead of constantly edging higher – would start to decrease; that, however, would undermine any investment activity. In essence, economic activity would grind to a total halt. In eurozone as a whole, we will now probably see a somewhat speedier price rise.
For euro as a world currency, the negative deposit interest will mean diminished attractiveness; therefore, its exchange rate towards other important currencies may be expected to fall. This is intentional, though. The ECB decision is just one among several rate-impacting factors; still, it clearly sets a trend and is not among the least important.